Move json to lib/json/{json.dart, json_frog.dart}

compiler/dartc.xml

 <project default="dist">
   <import file="sources.xml"/>
   <import file="test_sources.xml"/>
   <import file="corelib_sources.xml"/>
   <import file="compiler_corelib_sources.xml"/>
   <import file="domlib_sources.xml"/>
   <import file="htmllib_sources.xml"/>
   <import file="jsonlib_sources.xml"/>
 
   <!--
     The file build.properties does not exist. If provided, properties can be customized.
   -->
   <property file="build.properties" />
 
   <!--
     Configuration properties.
   -->
   <property name="build.dir" value="out"/>
 
   <property name="build.classes.dir" value="${build.dir}/classes"/>
 
   <property name="corelib.source.dir" value="${build.dir}/corelib"/>
 
   <property name="domlib.source.dir" value="${build.dir}/../client/dom/"/>
 
   <property name="htmllib.source.dir" value="${build.dir}/../client/html/"/>
 
-  <property name="jsonlib.source.dir" value="${build.dir}/../client/json/"/>
+  <property name="jsonlib.source.dir" value="${build.dir}/../lib/json/"/>
 
   <property name="build.test.classes.dir" value="${build.dir}/test/classes"/>
 
   <property name="dartc.jar" value="${build.dir}/dartc.jar"/>
 
   <property name="corelib.jar" value="${build.dir}/corelib.jar"/>
 
   <property name="domlib.jar" value="${build.dir}/domlib.jar"/>
 
   <property name="htmllib.jar" value="${build.dir}/htmllib.jar"/>
@@ skipping 212 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>