Make imports resynthesizing completely lazy.

pkg/analyzer/test/src/summary/resynthesize_test.dart

 // Copyright (c) 2015, 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 test.src.serialization.elements_test;
 
 import 'dart:convert';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/constant/value.dart';
@@ skipping 765 matching lines @@
           resynthesized.typeParameters[i],
           original.typeParameters[i],
           '$desc type parameter ${original.typeParameters[i].name}');
     }
   }
 
   void compareImportElements(ImportElementImpl resynthesized,
       ImportElementImpl original, String desc) {
     compareUriReferencedElements(resynthesized, original, desc);
     expect(resynthesized.importedLibrary.location,
-        original.importedLibrary.location);
-    expect(resynthesized.prefixOffset, original.prefixOffset);
+        original.importedLibrary.location,
+        reason: '$desc importedLibrary location');
+    expect(resynthesized.prefixOffset, original.prefixOffset,
+        reason: '$desc prefixOffset');
     if (original.prefix == null) {
-      expect(resynthesized.prefix, isNull);
+      expect(resynthesized.prefix, isNull, reason: '$desc prefix');
     } else {
       comparePrefixElements(
           resynthesized.prefix, original.prefix, original.prefix.name);
     }
-    expect(resynthesized.combinators.length, original.combinators.length);
+    expect(resynthesized.combinators.length, original.combinators.length,
+        reason: '$desc combinators');
     for (int i = 0; i < resynthesized.combinators.length; i++) {
       compareNamespaceCombinators(
           resynthesized.combinators[i], original.combinators[i]);
     }
   }
 
   void compareLabelElements(
       LabelElementImpl resynthesized, LabelElementImpl original, String desc) {
     expect(resynthesized.isOnSwitchMember, original.isOnSwitchMember,
         reason: desc);
@@ skipping 46 matching lines @@
       MethodElementImpl original, String desc) {
     // TODO(paulberry): do we need to deal with
     // MultiplyInheritedMethodElementImpl?
     compareExecutableElements(resynthesized, original, desc);
   }
 
   void compareNamespaceCombinators(
       NamespaceCombinator resynthesized, NamespaceCombinator original) {
     if (original is ShowElementCombinatorImpl &&
         resynthesized is ShowElementCombinatorImpl) {
-      expect(resynthesized.shownNames, original.shownNames);
-      expect(resynthesized.offset, original.offset);
-      expect(resynthesized.end, original.end);
+      expect(resynthesized.shownNames, original.shownNames,
+          reason: 'shownNames');
+      expect(resynthesized.offset, original.offset, reason: 'offset');
+      expect(resynthesized.end, original.end, reason: 'end');
     } else if (original is HideElementCombinatorImpl &&
         resynthesized is HideElementCombinatorImpl) {
-      expect(resynthesized.hiddenNames, original.hiddenNames);
+      expect(resynthesized.hiddenNames, original.hiddenNames,
+          reason: 'hiddenNames');
     } else if (resynthesized.runtimeType != original.runtimeType) {
       fail(
           'Type mismatch: expected ${original.runtimeType}, got ${resynthesized.runtimeType}');
     } else {
       fail('Unimplemented comparison for ${original.runtimeType}');
     }
   }
 
   void compareNamespaces(
       Namespace resynthesized, Namespace original, String desc) {
@@ skipping 3722 matching lines @@
       fail('Unexpectedly tried to get unlinked summary for $uri');
     }
     return serializedUnit;
   }
 
   @override
   bool hasLibrarySummary(String uri) {
     return true;
   }
 }