Place more guards for the cases which should be covered.

lib/compiler/implementation/unparse_validator.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.
 
 class ValidatorListener implements DiagnosticListener {
   final SourceFile sourceFile;
 
   ValidatorListener(this.sourceFile);
 
   void cancel([String reason, node, token, instruction, element]) {
@@ skipping 13 matching lines @@
 /**
  * Checks result's of [Node] unparse.
  */
 class UnparseValidator extends CompilerTask {
   final bool validateUnparse;
 
   String get name() => "Unparse validator";
 
   UnparseValidator(Compiler compiler, this.validateUnparse) : super(compiler);
 
-  void check(Element element) {
-    if (!validateUnparse) return;
-
-    // TODO(antonm): consider supporting other kinds of elements.
-    if (element is! PartialFunctionElement) return;
-
-    PartialFunctionElement originalFunction = element;
+  void checkFunction(PartialFunctionElement originalFunction) {
     FunctionExpression originalNode = originalFunction.parseNode(compiler);
     String unparsed = originalNode.unparse();
 
     Token newTokens = new StringScanner(unparsed).tokenize();
 
     // Find the getOrSet token.
     // TODO(ahe): This is to frigging complicated. Simplify it.
     Parser parser = new Parser(new Listener());
     Token getOrSet =
         parser.findGetOrSet(parser.parseModifiers(newTokens));
 
     // TODO(ahe): This is also too frigging complicated.
-    Script originalScript = element.getCompilationUnit().script;
+    Script originalScript = originalFunction.getCompilationUnit().script;
     String name = SourceSpan.withOffsets(
         originalFunction.beginToken, originalFunction.endToken,
         (beginOffset, endOffset) =>
             'synthesized:${originalScript.name}#$beginOffset:$endOffset');
     SourceFile synthesizedSourceFile = new SourceFile(name, unparsed);
     Script synthesizedScript =
         new Script(originalScript.uri, synthesizedSourceFile);
     LibraryElement lib = new LibraryElement(synthesizedScript);
-    lib.canUseNative = element.getLibrary().canUseNative;
+    lib.canUseNative = originalFunction.getLibrary().canUseNative;
     NodeListener listener =
         new NodeListener(new ValidatorListener(synthesizedSourceFile), lib);
     parser = new Parser(listener);
     parser.parseFunction(newTokens, getOrSet);
     FunctionExpression newNode = listener.popNode();
     // TODO(antonm): add Node comparison.
   }
+
+  void check(Element element) {
+    if (!validateUnparse) return;
+
+    if (element is PartialFunctionElement) {
+      checkFunction(element);
+    } else if (element.isGenerativeConstructor()) {
+      assert(element is FunctionElement);
+      // Generative constructors parse to very special function expressions.
+      // Handle them when classes are properly handled.
+    } else if (element.isField()) {
+      assert(element is VariableElement);
+      // Fields are just names with possible initialization expressions.
+      // Nothing to care about for now.
+    } else {
+      compiler.cancel('Cannot handle $element', element: element);
+    }
+  }
 }