Add a command-line option for generating source maps.

compiler/java/com/google/dart/compiler/CommandLineOptions.java

Index: compiler/java/com/google/dart/compiler/CommandLineOptions.java
===================================================================
--- compiler/java/com/google/dart/compiler/CommandLineOptions.java	(revision 471)
+++ compiler/java/com/google/dart/compiler/CommandLineOptions.java	(working copy)
@@ -66,6 +66,10 @@
         usage = "Classes to generate stubs\n (comma-separated list)")
     private String generateIsolateStubs = null;
 
+    @Option(name = "--generate_source_maps",
+        usage = "Generate source maps")
+    private boolean generateSourceMaps = false;
+
     @Option(name = "--human-readable-output",
         usage = "Write human readable javascript")
     private boolean generateHumanReadableOutput = false;
@@ -225,6 +229,10 @@
       return disableTypeOptimizations;
     }
 
+    public boolean generateSourceMaps() {
+      return generateSourceMaps;
+    }
+
     public boolean generateHumanReadableOutput() {
       return generateHumanReadableOutput;
     }