Adjust assertions in MethodMirror.source not to expect source for implicit constructors or signatur…

runtime/lib/mirrors.cc

Index: runtime/lib/mirrors.cc
diff --git a/runtime/lib/mirrors.cc b/runtime/lib/mirrors.cc
index 9f5c7634be0c0632f0e48c6c48bd960eb6d520c7..c2795a34a121c086ea3c521def925d2276aa2631 100644
--- a/runtime/lib/mirrors.cc
+++ b/runtime/lib/mirrors.cc
@@ -2037,6 +2037,9 @@ DEFINE_NATIVE_ENTRY(MethodMirror_return_type, 2) {
 DEFINE_NATIVE_ENTRY(MethodMirror_source, 1) {
   GET_NON_NULL_NATIVE_ARGUMENT(MirrorReference, ref, arguments->NativeArgAt(0));
   const Function& func = Function::Handle(ref.GetFunctionReferent());
+  if (func.IsImplicitConstructor() || func.IsSignatureFunction()) {

[hausner, 2014/02/07 21:07:58]

There may be other cases of functions that have no source code. I can think of
the forwarding constructors that are inserted in mixin application classes, or
implicit getter and setter functions, method extractor functions, no such method
dispatcher. Can these functions appear here?

[rmacnak, 2014/02/07 21:28:46]

Now it is considered an implicit constructor. Leaving a note that that might
change when the restrictions on mixins are relaxed.
No, we don't hand out mirrors on these.

+    return Instance::null();
+  }
   const Script& script = Script::Handle(func.script());
   const TokenStream& stream = TokenStream::Handle(script.tokens());
   const TokenStream::Iterator tkit(stream, func.end_token_pos());
@@ -2059,7 +2062,10 @@ DEFINE_NATIVE_ENTRY(MethodMirror_source, 1) {
        String::Handle(func.name()).Equals("<anonymous closure>"))) {  // Case 3.
     last_tok_len = 0;
   }
-  return script.GetSnippet(from_line, from_col, to_line, to_col + last_tok_len);
+  const Instance& result = Instance::Handle(
+      script.GetSnippet(from_line, from_col, to_line, to_col + last_tok_len));
+  ASSERT(!result.IsNull());
+  return result.raw();
 }