Don't throw an exception when model is changed.

pkg/analyzer/test/generated/incremental_resolver_test.dart

 // Copyright (c) 2014, 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.
 
 library engine.incremental_resolver_test;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
@@ skipping 2099 matching lines @@
         code.substring(offset + edit.length);
     CompilationUnit newUnit = _parseUnit(newCode);
     // update tokens
     {
       int delta = edit.replacement.length - edit.length;
       _shiftTokens(unit.beginToken, offset, delta);
     }
     // replace the node
     AstNode oldNode = _findNodeAt(unit, offset, predicate);
     AstNode newNode = _findNodeAt(newUnit, offset, predicate);
-    bool success = NodeReplacer.replace(oldNode, newNode);
-    expect(success, isTrue);
+    {
+      bool success = NodeReplacer.replace(oldNode, newNode);
+      expect(success, isTrue);
+    }
     // do incremental resolution
     IncrementalResolver resolver = new IncrementalResolver(
         typeProvider,
         unit.element,
         edit.offset,
         edit.length,
         edit.replacement.length);
-    resolver.resolve(newNode);
+    bool success = resolver.resolve(newNode);
+    expect(success, isTrue);
     // resolve "newCode" from scratch
     CompilationUnit fullNewUnit;
     {
       source = addSource(newCode);
       LibraryElement library = resolve(source);
       fullNewUnit = resolveCompilationUnit(source, library);
     }
     _SameResolutionValidator.assertSameResolution(unit, fullNewUnit);
   }
 
@@ skipping 1828 matching lines @@
     _visitList(node.metadata, other.metadata);
     _visitNode(node.identifier, other.identifier);
   }
 
   static void assertSameResolution(CompilationUnit actual,
       CompilationUnit expected) {
     _SameResolutionValidator validator = new _SameResolutionValidator(expected);
     actual.accept(validator);
   }
 }