Adds typechecking of return values

frog/value.dart

 // Copyright (c) 2011, 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.
 
 /**
  * Represents a meta-value for code generation.
  */
 class Value {
   /** The [Type] of the [Value]. */
   Type type;
@@ skipping 15 matching lines @@
 
   Value(this.type, this.code, this.span,
         // TODO(sigmund): reorder, so that needsTemp comes first.
         [this.isSuper = false, this.needsTemp = true, this.isType = false]) {
     if (type == null) type = world.varType;
   }
 
   /** Is this value a constant expression? */
   bool get isConst() => false;
 
+  /**
+   * A canonicalized form of the code. Two const expressions that result in the
+   * same instance should have the same [canonicalCode].
+   */
+  String get canonicalCode() => null;
+
   // TODO(jimhug): Fix these names once get/set are truly pseudo-keywords.
   //   See issue #379.
   Value get_(MethodGenerator context, String name, Node node) {
     final member = _resolveMember(context, name, node);
     if (member != null) {
       return member._get(context, node, this);
     } else {
       return invokeNoSuchMethod(context, 'get:$name', node);
     }
   }
@@ skipping 274 matching lines @@
     // TODO(jmesserly): better assert for integers?
     if (toType.isNum) toType = world.numType;
 
     // Generate a check like these:
     //   obj && obj.is$TypeName()
     //   $assert_int(obj)
     //
     // We rely on the fact that calling an undefined method produces a JS
     // TypeError. Alternatively we could define fallbacks on Object that throw.
     String check;
-    if (toType.library.isCore && toType.typeofName != null) {
+    if (toType.isVoid) {
+      check = '\$assert_void($code)';
+      if (toType.typeCheckCode == null) {
+        toType.typeCheckCode = '''
+function \$assert_void(x) {
+  return x == null ? x : x.is\$void(); // throws TypeError
+}''';
+      }
+    } else if (toType.library.isCore && toType.typeofName != null) {
       check = '\$assert_${toType.name}($code)';
 
       if (toType.typeCheckCode == null) {
         toType.typeCheckCode = '''
 function \$assert_${toType.name}(x) {
   if (x == null || typeof(x) == "${toType.typeofName}") return x;
   throw new TypeError("'" + x + "' is not a ${toType.name}.");
 }''';
       }
     } else {
@@ skipping 323 matching lines @@
 
     // Then look for members in my library.
     member = home.library.lookup(name, span);
     if (member != null) {
       return member;
     }
 
     return null;
   }
 }