Update Dart install instructions

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[INFO] Building Nuxeo API Playground 9.3-SNAPSHOT
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[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ nuxeo-api-playground ---
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[INFO] --- maven-enforcer-plugin:1.4.1:enforce (default) @ nuxeo-api-playground ---
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[INFO] --- buildnumber-maven-plugin:1.2:create (default) @ nuxeo-api-playground ---
[INFO] Storing buildNumber: 20170810-123835 at timestamp: 1502361515548
[INFO] Executing: /bin/sh -c cd /home/anahide/ws/nuxeo-new/addons/nuxeo-api-playground && git rev-parse --verify HEAD
[INFO] Working directory: /home/anahide/ws/nuxeo-new/addons/nuxeo-api-playground
[INFO] Storing buildScmBranch: UNKNOWN
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[INFO] --- incremental-build-plugin:1.6-NX1:incremental-build (default) @ nuxeo-api-playground ---
[INFO] Verifying module descriptor ...
[INFO] Pom descriptor modification detected.
[INFO] Deleting /home/anahide/ws/nuxeo-new/addons/nuxeo-api-playground/target
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[INFO] --- maven-resources-plugin:2.6:copy-resources (copy-readme) @ nuxeo-api-playground ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] Copying 1 resource
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[INFO] --- maven-resources-plugin:2.6:copy-resources (copy-readme-parent) @ nuxeo-api-playground ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] Copying 0 resource
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[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ nuxeo-api-playground ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] Copying 2 resources
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[INFO] --- maven-antrun-plugin:1.7:run (gulp-build) @ nuxeo-api-playground ---
[INFO] Executing tasks

main:

init:
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[INFO] Reactor Summary:
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[INFO] Nuxeo Web UI - Web Application ..................... SUCCESS [02:40 min]
[INFO] Nuxeo API Playground ............................... FAILURE [  0.115 s]
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[INFO] BUILD FAILURE
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[INFO] Total time: 03:03 min
[INFO] Finished at: 2017-08-10T12:38:35+02:00
[INFO] Final Memory: 835M/2081M
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[ERROR] Failed to execute goal org.apache.maven.plugins:maven-antrun-plugin:1.7:run (gulp-build) on project nuxeo-api-playground: An Ant BuildException has occured: The following error occurred while executing this line:
[ERROR] /home/anahide/ws/nuxeo-new/addons/nuxeo-api-playground/src/main/dart/build.xml:4: Execute failed: java.io.IOException: Cannot run program "pub" (in directory "/home/anahide/ws/nuxeo-new/addons/nuxeo-api-playground/src/main/dart"): error=2, No such file or directory
[ERROR] around Ant part ...<ant antfile="/home/anahide/ws/nuxeo-new/addons/nuxeo-api-playground/src/main/dart/build.xml" dir="/home/anahide/ws/nuxeo-new/addons/nuxeo-api-playground/src/main/dart"></ant>... @ 4:172 in /home/anahide/ws/nuxeo-new/addons/nuxeo-api-playground/target/antrun/build-main.xml
[ERROR] -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
[ERROR]
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <goals> -rf :nuxeo-api-playground