Report compile-time error if using `this` implicitly in initializers

pkg/compiler/lib/src/resolution/members.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.
 
 part of resolution;
 
 abstract class TreeElements {
   AnalyzableElement get analyzedElement;
   Iterable<Node> get superUses;
 
@@ skipping 2094 matching lines @@
  * Do not subclass or instantiate this class outside this library
  * except for testing.
  */
 class ResolverVisitor extends MappingVisitor<ResolutionResult> {
   /**
    * The current enclosing element for the visited AST nodes.
    *
    * This field is updated when nested closures are visited.
    */
   Element enclosingElement;
+
+  /// Whether we are in a context where `this` is accessible (this will be false
+  /// in static contexts, factory methods, and field initializers).
   bool inInstanceContext;
   bool inCheckContext;
   bool inCatchBlock;
 
   Scope scope;
   ClassElement currentClass;
   ExpressionStatement currentExpressionStatement;
   bool sendIsMemberAccess = false;
   StatementScope statementScope;
   int allowedCategory = ElementCategory.VARIABLE | ElementCategory.FUNCTION
@@ skipping 172 matching lines @@
         // Set the type to be `dynamic` to mark that this is a type literal.
         registry.setType(node, const DynamicType());
       }
       element = reportLookupErrorIfAny(element, node, node.source);
       if (element == null) {
         if (!inInstanceContext) {
           element = warnAndCreateErroneousElement(
               node, node.source, MessageKind.CANNOT_RESOLVE,
               {'name': node});
           registry.registerThrowNoSuchMethod();
+
+          // We also report an error within initializers because `this` is
+          // implicitly accessed when unqualified identifiers are not resolved.
+          // For details, see section 16.14.3 of the spec (2nd edition):
+          //   An unqualified invocation `i` of the form `id(a1, ...)`
+          //   ...
+          //   If `i` does not occur inside a top level or static function, `i`
+          //   is equivalent to `this.id(a1 , ...)`.
+          if ((enclosingElement.isInstanceMember && enclosingElement.isField) ||
+              (enclosingElement.isGenerativeConstructor)) {
+            compiler.reportError(

[Johnni Winther, 2015/04/10 07:55:50]

From a users perspective, it might be better to have a special message that
combines the information from CANNOT_RESOLVE and NO_INSTANCE_AVAILABLE for this
situation.

I you want to do that, you could add a named argument {bool reportError: false}
to [warnAndCreateErroneousElement] (and maybe name it to
`reportAndCreateErroneousElement`) and use the `reportError` to either call
`compiler.reportWarning` or `compiler.reportError`.

[Siggi Cherem (dart-lang), 2015/04/10 18:56:26]

I have it a try. WDYT?

+                node, MessageKind.NO_INSTANCE_AVAILABLE, {'name': node.source});
+          }
         }
       } else if (element.isErroneous) {
         // Use the erroneous element.
       } else {
         if ((element.kind.category & allowedCategory) == 0) {
           element = warnAndCreateErroneousElement(
               node, name,
               MessageKind.GENERIC,
               // TODO(ahe): Improve error message. Need UX input.
               {'text': "is not an expression $element"});
@@ skipping 2760 matching lines @@
 }
 
 /// The result for the resolution of the `assert` method.
 class AssertResult implements ResolutionResult {
   const AssertResult();
 
   Element get element => null;
 
   String toString() => 'AssertResult()';
 }