Ok, I've tracked down the problem.
The problem is that gwt-dev somehow relies on gson 1.7.2 but should rely on 
2.6.2. I dont know what the problem is. I hope Jens or Thomas can help with 
that.
The problem was that gson 1.7.2 incorrectly parsed provided source map 
string and returned empty JsonObject which resulted in NPE.

[INFO] --- maven-dependency-plugin:2.8:tree (default-cli) @ sourcemap ---
[INFO] com.testcase:sourcemap:war:1.0-SNAPSHOT
[INFO] +- com.google.gwt:gwt-user:jar:2.8.0-rc2:provided
[INFO] |  +- com.google.jsinterop:jsinterop-annotations:jar:1.0.0:provided
[INFO] |  +- 
com.google.jsinterop:jsinterop-annotations:jar:sources:1.0.0:provided
[INFO] |  +- javax.validation:validation-api:jar:1.0.0.GA:provided
[INFO] |  +- javax.validation:validation-api:jar:sources:1.0.0.GA:provided
[INFO] |  +- javax.servlet:javax.servlet-api:jar:3.1.0:provided
[INFO] |  \- org.w3c.css:sac:jar:1.3:provided
[INFO] \- com.google.gwt:gwt-dev:jar:2.8.0-rc2:provided
[INFO]    +- com.google.code.gson:gson:jar:1.7.2:provided (version managed from 
2.6.2)
[INFO]    +- org.ow2.asm:asm:jar:5.0.3:provided
[INFO]    +- org.ow2.asm:asm-util:jar:5.0.3:provided
[INFO]    |  \- org.ow2.asm:asm-tree:jar:5.0.3:provided
[INFO]    |     \- (org.ow2.asm:asm:jar:5.0.3:provided - omitted for duplicate)
[INFO]    +- org.ow2.asm:asm-commons:jar:5.0.3:provided
[INFO]    |  \- (org.ow2.asm:asm-tree:jar:5.0.3:provided - omitted for 
duplicate)
[INFO]    +- colt:colt:jar:1.2.0:provided
[INFO]    +- ant:ant:jar:1.6.5:provided
[INFO]    +- commons-collections:commons-collections:jar:3.2.2:provided
[INFO]    +- commons-io:commons-io:jar:2.4:provided
[INFO]    +- com.ibm.icu:icu4j:jar:50.1.1:provided
[INFO]    +- tapestry:tapestry:jar:4.0.2:provided
[INFO]    +- net.sourceforge.htmlunit:htmlunit:jar:2.19:provided
[INFO]    |  +- xalan:xalan:jar:2.7.1:provided (version managed from 2.7.2)
[INFO]    |  |  \- xalan:serializer:jar:2.7.1:provided
[INFO]    |  |     \- xml-apis:xml-apis:jar:1.4.01:provided (version managed 
from 1.3.04)
[INFO]    |  +- (commons-collections:commons-collections:jar:3.2.2:provided - 
version managed from 3.2.1; omitted for duplicate)
[INFO]    |  +- org.apache.commons:commons-lang3:jar:3.1:provided (version 
managed from 3.4)
[INFO]    |  +- org.apache.httpcomponents:httpclient:jar:4.5:provided (version 
managed from 4.5.1)
[INFO]    |  |  +- org.apache.httpcomponents:httpcore:jar:4.4.1:provided
[INFO]    |  |  \- (commons-codec:commons-codec:jar:1.10:provided - version 
managed from 1.9; omitted for duplicate)
[INFO]    |  +- org.apache.httpcomponents:httpmime:jar:4.5.1:provided
[INFO]    |  |  \- (org.apache.httpcomponents:httpclient:jar:4.5:provided - 
version managed from 4.5.1; omitted for duplicate)
[INFO]    |  +- commons-codec:commons-codec:jar:1.10:provided
[INFO]    |  +- net.sourceforge.htmlunit:htmlunit-core-js:jar:2.17:provided
[INFO]    |  +- xerces:xercesImpl:jar:2.11.0.SP4:provided (version managed from 
2.11.0)
[INFO]    |  +- net.sourceforge.nekohtml:nekohtml:jar:1.9.22:provided
[INFO]    |  +- net.sourceforge.cssparser:cssparser:jar:0.9.18:provided
[INFO]    |  |  \- (org.w3c.css:sac:jar:1.3:provided - omitted for duplicate)
[INFO]    |  +- (commons-io:commons-io:jar:2.4:provided - omitted for duplicate)
[INFO]    |  +- commons-logging:commons-logging:jar:1.1.1:provided (version 
managed from 1.2)
[INFO]    |  \- 
org.eclipse.jetty.websocket:websocket-client:jar:9.2.13.v20150730:provided
[INFO]    |     +- (org.eclipse.jetty:jetty-util:jar:9.2.14.v20151106:provided 
- version managed from 9.2.13.v20150730; omitted for duplicate)
[INFO]    |     +- (org.eclipse.jetty:jetty-io:jar:9.2.14.v20151106:provided - 
version managed from 9.2.13.v20150730; omitted for duplicate)
[INFO]    |     \- 
org.eclipse.jetty.websocket:websocket-common:jar:9.2.13.v20150730:provided
[INFO]    |        +- 
org.eclipse.jetty.websocket:websocket-api:jar:9.2.13.v20150730:provided
[INFO]    |        +- 
(org.eclipse.jetty:jetty-util:jar:9.2.14.v20151106:provided - version managed 
from 9.2.13.v20150730; omitted for duplicate)
[INFO]    |        \- (org.eclipse.jetty:jetty-io:jar:9.2.14.v20151106:provided 
- version managed from 9.2.13.v20150730; omitted for duplicate)
[INFO]    +- org.eclipse.jetty:jetty-webapp:jar:9.2.14.v20151106:provided
[INFO]    |  +- org.eclipse.jetty:jetty-xml:jar:9.2.14.v20151106:provided
[INFO]    |  |  \- (org.eclipse.jetty:jetty-util:jar:9.2.14.v20151106:provided 
- version managed from 9.2.13.v20150730; omitted for duplicate)
[INFO]    |  \- org.eclipse.jetty:jetty-servlet:jar:9.2.14.v20151106:provided
[INFO]    |     \- 
org.eclipse.jetty:jetty-security:jar:9.2.14.v20151106:provided
[INFO]    |        \- 
(org.eclipse.jetty:jetty-server:jar:9.2.14.v20151106:provided - omitted for 
duplicate)
[INFO]    +- org.eclipse.jetty:jetty-servlets:jar:9.2.14.v20151106:provided
[INFO]    |  +- 
org.eclipse.jetty:jetty-continuation:jar:9.2.14.v20151106:provided
[INFO]    |  +- org.eclipse.jetty:jetty-http:jar:9.2.14.v20151106:provided
[INFO]    |  |  \- (org.eclipse.jetty:jetty-util:jar:9.2.14.v20151106:provided 
- version managed from 9.2.13.v20150730; omitted for duplicate)
[INFO]    |  +- org.eclipse.jetty:jetty-util:jar:9.2.14.v20151106:provided
[INFO]    |  \- org.eclipse.jetty:jetty-io:jar:9.2.14.v20151106:provided
[INFO]    |     \- (org.eclipse.jetty:jetty-util:jar:9.2.14.v20151106:provided 
- version managed from 9.2.13.v20150730; omitted for duplicate)
[INFO]    +- org.eclipse.jetty:jetty-annotations:jar:9.2.14.v20151106:provided
[INFO]    |  +- org.eclipse.jetty:jetty-plus:jar:9.2.14.v20151106:provided
[INFO]    |  |  +- 
(org.eclipse.jetty:jetty-webapp:jar:9.2.14.v20151106:provided - omitted for 
duplicate)
[INFO]    |  |  \- org.eclipse.jetty:jetty-jndi:jar:9.2.14.v20151106:provided
[INFO]    |  |     \- 
(org.eclipse.jetty:jetty-util:jar:9.2.14.v20151106:provided - version managed 
from 9.2.13.v20150730; omitted for duplicate)
[INFO]    |  +- (org.eclipse.jetty:jetty-webapp:jar:9.2.14.v20151106:provided - 
omitted for duplicate)
[INFO]    |  +- javax.annotation:javax.annotation-api:jar:1.2:provided
[INFO]    |  +- (org.ow2.asm:asm:jar:5.0.3:provided - version managed from 
5.0.1; omitted for duplicate)
[INFO]    |  \- (org.ow2.asm:asm-commons:jar:5.0.1:provided - omitted for 
conflict with 5.0.3)
[INFO]    \- org.eclipse.jetty:apache-jsp:jar:9.2.14.v20151106:provided
[INFO]       +- (org.eclipse.jetty:jetty-util:jar:9.2.14.v20151106:provided - 
version managed from 9.2.13.v20150730; omitted for duplicate)
[INFO]       +- org.eclipse.jetty:jetty-server:jar:9.2.14.v20151106:provided
[INFO]       |  +- (javax.servlet:javax.servlet-api:jar:3.1.0:provided - 
omitted for duplicate)
[INFO]       |  +- (org.eclipse.jetty:jetty-http:jar:9.2.14.v20151106:provided 
- omitted for duplicate)
[INFO]       |  \- (org.eclipse.jetty:jetty-io:jar:9.2.14.v20151106:provided - 
version managed from 9.2.13.v20150730; omitted for duplicate)
[INFO]       +- org.eclipse.jetty.toolchain:jetty-schemas:jar:3.1.M0:provided
[INFO]       +- (javax.servlet:javax.servlet-api:jar:3.1.0:provided - omitted 
for duplicate)
[INFO]       \- org.mortbay.jasper:apache-jsp:jar:8.0.9.M3:provided
[INFO]          +- 
(org.eclipse.jetty.toolchain:jetty-schemas:jar:3.1.M0:provided - omitted for 
duplicate)
[INFO]          \- org.mortbay.jasper:apache-el:jar:8.0.9.M3:provided


On Friday, August 19, 2016 at 2:33:55 PM UTC+3, Alberto Mancini wrote:
>
>
>
> mvn:run should give you the error 
>>
>
> Sorry, i mean:  
> mvn gwt:run 
>
>  
> Thanks,  
>    A. 
>

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