Ensure dart_analyzer is built to run on a 1.6+ VM.

compiler/build.xml

 <!--
   Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
   for details. All rights reserved. Use of this source code is governed by a
   BSD-style license that can be found in the LICENSE file.
  -->
 <project default="dist">
   <!--
       EDIT DARTC.XML FIRST.  This file is no longer used to build
       dartc proper.  It is only here to avoid breaking dartium.gyp
       short term.  Change dartc.xml to get everything working, 
@@ skipping 59 matching lines @@
     <!--
       Ensure the necessary subdirectories exist.
     -->
     <mkdir dir="${build.classes.dir}"/>
 
     <javac destdir="${build.classes.dir}"
            sourcepath=""
            srcdir="java"
            includes="${java_sources}"
            fork="true"
-           debug="true">
+           debug="true"
+           source="1.6"
+           target="1.6">
       <classpath refid="classpath.compile"/>
     </javac>
     <!--
       Copy all non-java resources.
     -->
     <copy todir="${build.classes.dir}">
       <filelist refid="java_resources"/>
     </copy>
 
   </target>
 
   <target name="compile-tests" depends="compile" description="Compiles all of the java tests and copies the resources to the test classes directory." >
     <mkdir dir="${build.test.classes.dir}"/>
     <javac destdir="${build.test.classes.dir}"
            sourcepath=""
            srcdir="javatests"
            includes="${javatests_sources}"
            excludes="com/google/dart/compiler/vm/**"
            fork="true"
-           debug="true">
+           debug="true"
+           source="1.6"
+           target="1.6">
       <classpath refid="classpath.compile.tests"/>
     </javac>
     <!--
       Copy the non-java resources.
     -->
     <copy todir="${build.test.classes.dir}">
       <filelist refid="javatests_resources"/>
     </copy>
   </target>
 
@@ skipping 59 matching lines @@
 
   <target name="clean" description="Deletes the build output directory.">
     <delete dir="${build.dir}"/>
     <delete dir="${dist.dir}"/>
   </target>
 
   <target name="tests.jar" depends="compile-tests">
     <jar destfile="${build.dir}/tests.jar" basedir="${build.test.classes.dir}"/>
   </target>
 </project>