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

runtime/lib/mirrors_impl.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // VM-specific implementation of the dart:mirrors library.
 
 import "dart:collection";
 
 final emptyList = new UnmodifiableListView([]);
 final emptyMap = new _UnmodifiableMapView({});
@@ skipping 1341 matching lines @@
         }
       }
     }
     return _constructorName;
   }
 
   String _source = null;
   String get source {
     if (_source == null) {
       _source = _MethodMirror_source(_reflectee);
-      assert(_source != null);
     }
     return _source;
   }
 
   void _patchConstructorName(ownerName) {
     var cn = _n(constructorName);
     if(cn == ''){
       _simpleName = _s(ownerName);
     } else {
       _simpleName = _s(ownerName + "." + cn);
@@ skipping 208 matching lines @@
     if (typeMirror == null) {
       typeMirror = makeLocalTypeMirror(key);
       _instanitationCache[key] = typeMirror;
       if (typeMirror is ClassMirror && !typeMirror._isGeneric) {
         _declarationCache[key] = typeMirror;
       }
     }
     return typeMirror;
   }
 }