Skip spurious constants in deferred computation.

pkg/compiler/lib/src/deferred_load.dart

 // Copyright (c) 2014, 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.
 
 library deferred_load;
 
 import 'common/backend_api.dart' show Backend;
 import 'common/tasks.dart' show CompilerTask;
 import 'common.dart';
 import 'compiler.dart' show Compiler;
@@ skipping 318 matching lines @@
             }),
             IMPACT_USE);
 
         if (analyzableElement.resolvedAst.kind != ResolvedAstKind.PARSED) {
           return;
         }
 
         TreeElements treeElements = analyzableElement.resolvedAst.elements;
         assert(treeElements != null);
 
+        // TODO(johnniwinther): Add only expressions that are actually needed.
+        // Currently we have some noise here: Some potential expressions are
+        // seen that should never be added (for instance field initializers
+        // in constant constructors, like `this.field = parameter`). And some
+        // implicit constant expression are seen that we should be able to add
+        // (like primitive constant literals like `true`, `"foo"` and `0`).
+        // See dartbug.com/26406 for context.
         treeElements
             .forEachConstantNode((Node node, ConstantExpression expression) {
           // Explicitly depend on the backend constants.
-          ConstantValue value;
-          value = backend.constants.getConstantValue(expression);
-          // TODO(johnniwinther): ensure `value` is not null or use directly the
-          // expression to calculate dependencies. See dartbug.com/26406 for
-          // context.
-          if (value != null) {
+          if (backend.constants.hasConstantValue(expression)) {
+            ConstantValue value =
+                backend.constants.getConstantValue(expression);
+            assert(invariant(node, value != null,
+                message: "Constant expression without value: "
+                         "${expression.toStructuredText()}."));
             constants.add(value);
+          } else {
+            assert(invariant(node,
+                expression.isImplicit || expression.isPotential,
+                message: "Unexpected unevaluated constant expression: "
+                    "${expression.toStructuredText()}."));
           }
         });
       }
     }
 
     // TODO(sigurdm): How is metadata on a patch-class handled?
     for (MetadataAnnotation metadata in element.metadata) {
       ConstantValue constant =
           backend.constants.getConstantValueForMetadata(metadata);
       if (constant != null) {
@@ skipping 627 matching lines @@
     return result;
   }
 
   bool operator ==(other) {
     if (other is! _DeclaredDeferredImport) return false;
     return declaration == other.declaration;
   }
 
   int get hashCode => declaration.hashCode * 17;
 }