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..d82cf7d2bc984f27df5c7e05a9c34e73fde0d89f 100644
--- a/runtime/lib/mirrors.cc
+++ b/runtime/lib/mirrors.cc
@@ -2037,6 +2037,13 @@ 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()) {
+    // We may need to handle more cases when the restrictions on mixins are
+    // relaxed. In particular we might start associating some source with the
+    // forwarding constructors when it becomes possible to specify a particular
+    // constructor from the mixin to use.
+    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 +2066,10 @@ 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();
 }