Fix precompile build issue with IsString change.

runtime/vm/symbols.cc

 // 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.
 
 #include "vm/symbols.h"
 
 #include "vm/handles.h"
 #include "vm/handles_impl.h"
 #include "vm/hash_table.h"
 #include "vm/isolate.h"
@@ skipping 387 matching lines @@
   // 3. Walk the heap and build a new table from surviving symbols.
   GrowableArray<String*> symbols;
   class SymbolCollector : public ObjectVisitor {
    public:
     SymbolCollector(Thread* thread,
                     GrowableArray<String*>* symbols)
         : symbols_(symbols),
           zone_(thread->zone()) {}
 
     void VisitObject(RawObject* obj) {
-      if (obj->IsString() && obj->IsCanonical()) {
+      if (obj->IsCanonical() && obj->IsStringInstance()) {
         symbols_->Add(&String::ZoneHandle(zone_, String::RawCast(obj)));
       }
     }
 
    private:
     GrowableArray<String*>* symbols_;
     Zone* zone_;
   };
 
   SymbolCollector visitor(Thread::Current(), &symbols);
@@ skipping 352 matching lines @@
 RawObject* Symbols::GetVMSymbol(intptr_t object_id) {
   ASSERT(IsVMSymbolId(object_id));
   intptr_t i = (object_id - kMaxPredefinedObjectIds);
   if ((i > kIllegal) && (i < Symbols::kMaxPredefinedId)) {
     return symbol_handles_[i]->raw();
   }
   return Object::null();
 }
 
 }  // namespace dart