Add support for javascript incompatibility warnings.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 35921)
+++ runtime/vm/parser.cc	(working copy)
@@ -9002,6 +9002,7 @@
                          String::Handle(field.name()).ToCString());
         } else {
           isolate()->long_jump_base()->Jump(1, error);
+          UNREACHABLE();
         }
       }
       ASSERT(const_value.IsNull() || const_value.IsInstance());
@@ -10297,14 +10298,16 @@
   interpolate_arg.SetAt(0, value_arr);
 
   // Call interpolation function.
-  String& concatenated = String::ZoneHandle(isolate());
+  Object& result = Object::Handle(isolate());
   {
     PAUSETIMERSCOPE(isolate(), time_compilation);
-    concatenated ^= DartEntry::InvokeFunction(func, interpolate_arg);
+    result = DartEntry::InvokeFunction(func, interpolate_arg);
   }
-  if (concatenated.IsUnhandledException()) {
-    ErrorMsg("Exception thrown in Parser::Interpolate");
+  if (result.IsUnhandledException()) {
+    ErrorMsg("%s", Error::Cast(result).ToErrorCString());
   }
+  String& concatenated = String::ZoneHandle(isolate());
+  concatenated ^= result.raw();
   concatenated = Symbols::New(concatenated);
   return concatenated;
 }