Move symbol demangling to dart:_internal to remove dart:core dependency on dart:mirrors.

runtime/lib/mirrors_impl.dart

Index: runtime/lib/mirrors_impl.dart
diff --git a/runtime/lib/mirrors_impl.dart b/runtime/lib/mirrors_impl.dart
index c2476371f57ce4b4ea78f1f093097569c9a32e47..86b7494c62c63e3a7262b5f7a998c943d8963b20 100644
--- a/runtime/lib/mirrors_impl.dart
+++ b/runtime/lib/mirrors_impl.dart
@@ -15,11 +15,11 @@ class _InternalMirrorError {
   String toString() => _msg;
 }
 
-String _n(Symbol symbol) => _symbol_dev.Symbol.getName(symbol);
+String _n(Symbol symbol) => internal.Symbol.getName(symbol);
 
 Symbol _s(String name) {
   if (name == null) return null;
-  return new _symbol_dev.Symbol.unvalidated(name);
+  return new internal.Symbol.unvalidated(name);
 }
 
 Symbol _computeQualifiedName(DeclarationMirror owner, Symbol simpleName) {