Only allow 'void' as a return type.

lib/compiler/implementation/compiler.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.
 
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
 const bool REPORT_EXCESS_RESOLUTION = false;
@@ skipping 715 matching lines @@
       if (phase == PHASE_COMPILING) {
         log('Compiled ${codegenWorld.generatedCode.length} methods.');
       } else {
         log('Recompiled ${world.recompilationCandidates.processed} methods.');
       }
       progress.reset();
     }
     backend.codegen(work);
   }
 
-  DartType resolveTypeAnnotation(Element element, TypeAnnotation annotation) {
+  DartType resolveTypeAnnotation(Element element,
+                                 TypeAnnotation annotation) {
     return resolver.resolveTypeAnnotation(element, annotation);
   }
 
+  DartType resolveReturnType(Element element,
+                             TypeAnnotation annotation) {
+    return resolver.resolveReturnType(element, annotation);
+  }
+
   FunctionSignature resolveSignature(FunctionElement element) {
     return withCurrentElement(element,
                               () => resolver.resolveSignature(element));
   }
 
   FunctionSignature resolveFunctionExpression(Element element,
                                               FunctionExpression node) {
     return withCurrentElement(element,
         () => resolver.resolveFunctionExpression(element, node));
   }
@@ skipping 188 matching lines @@
     final endOffset = end.charOffset + end.slowCharCount;
 
     // [begin] and [end] might be the same for the same empty token. This
     // happens for instance when scanning '$$'.
     assert(endOffset >= beginOffset);
     return f(beginOffset, endOffset);
   }
 
   String toString() => 'SourceSpan($uri, $begin, $end)';
 }