Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/frog/frog_options.dart

Index: dart/frog/frog_options.dart
diff --git a/dart/frog/frog_options.dart b/dart/frog/frog_options.dart
index 041416c6d157fb889c0f014eb76431514ebddc67..46e1020b14a05015d3d5de0866d2dade761e4dbd 100644
--- a/dart/frog/frog_options.dart
+++ b/dart/frog/frog_options.dart
@@ -27,6 +27,7 @@ class FrogOptions {
 
   // Options that modify behavior significantly
   bool legOnly = false;
+  bool allowMockCompilation = false;

[ngeoffray, 2012/03/22 10:04:39]

Good catch :)

   bool enableAsserts = false;
   bool enableTypeChecks = false;
   bool warningsAsErrors = false;
@@ -115,6 +116,10 @@ class FrogOptions {
         legOnly = true;
         return true;
 
+      case '--allow-mock-compilation':
+        allowMockCompilation = true;
+        return true;
+
       case '--enable-asserts':
         enableAsserts = true;
         return true;