A method or accessor with empty body is implicitly abstract.

dart/lib/compiler/implementation/elements/elements.dart

Index: dart/lib/compiler/implementation/elements/elements.dart
diff --git a/dart/lib/compiler/implementation/elements/elements.dart b/dart/lib/compiler/implementation/elements/elements.dart
index 5da84bcafd882921018d3d6ec859a86f0c1fb303..82808cf25c6febaf26d3acf80ab81858a944c238 100644
--- a/dart/lib/compiler/implementation/elements/elements.dart
+++ b/dart/lib/compiler/implementation/elements/elements.dart
@@ -353,6 +353,8 @@ class Element implements Spannable {
   FunctionElement asFunctionElement() => null;
 
   static bool isInvalid(Element e) => e == null || e.isErroneous();
+
+  bool isAbstract(Compiler compiler) => modifiers.isAbstract();
 }
 
 /**
@@ -1184,7 +1186,7 @@ class FunctionElement extends Element {
     return type;
   }
 
-  Node parseNode(DiagnosticListener listener) {
+  FunctionExpression parseNode(DiagnosticListener listener) {
     if (patch == null) {
       if (modifiers.isExternal()) {
         listener.cancel("Compiling external function with no implementation.",
@@ -1207,6 +1209,15 @@ class FunctionElement extends Element {
       return super.toString();
     }
   }
+
+  bool isAbstract(Compiler compiler) {
+    if (super.isAbstract(compiler)) return true;
+    if (modifiers.isExternal()) return false;
+    if (isFunction() || isAccessor()) {
+      return !parseNode(compiler).hasBody();
+    }
+    return false;
+  }
 }
 
 class ConstructorBodyElement extends FunctionElement {