Resynthesize most of the ImportElementImpl lazily.

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 1064 matching lines @@
       fail('Type mismatch: expected ${original.runtimeType},'
           ' got ${resynthesized.runtimeType} ($desc)');
     } else {
       fail('Unimplemented comparison for ${original.runtimeType}');
     }
   }
 
   void compareUriReferencedElements(UriReferencedElementImpl resynthesized,
       UriReferencedElementImpl original, String desc) {
     compareElements(resynthesized, original, desc);
-    expect(resynthesized.uri, original.uri);
-    expect(resynthesized.uriOffset, original.uriOffset, reason: desc);
-    expect(resynthesized.uriEnd, original.uriEnd, reason: desc);
+    expect(resynthesized.uri, original.uri, reason: '$desc uri');
+    expect(resynthesized.uriOffset, original.uriOffset,
+        reason: '$desc uri uriOffset');
+    expect(resynthesized.uriEnd, original.uriEnd, reason: '$desc uriEnd');
   }
 
   void compareVariableElements(
       VariableElement resynthesized, VariableElement original, String desc) {
     compareElements(resynthesized, original, desc);
     compareTypes(resynthesized.type, original.type, desc);
     VariableElementImpl resynthesizedActual =
         getActualElement(resynthesized, desc);
     VariableElementImpl originalActual = getActualElement(original, desc);
     if (!skipBrokenAstInference) {
@@ skipping 3500 matching lines @@
       fail('Unexpectedly tried to get unlinked summary for $uri');
     }
     return serializedUnit;
   }
 
   @override
   bool hasLibrarySummary(String uri) {
     return true;
   }
 }