Report error if the same uri appears in multiple summaries.

pkg/dev_compiler/lib/src/compiler/compiler.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:collection' show HashSet, Queue;
 import 'dart:convert' show JSON;
 import 'dart:io' show File;
 
 import 'package:analyzer/analyzer.dart'
     show AnalysisError, CompilationUnit, ErrorSeverity;
@@ skipping 71 matching lines @@
         options: options.contextBuilderOptions);
 
     var analysisOptions = contextBuilder.getAnalysisOptions(analysisRoot);
     var sdk = contextBuilder.findSdk(null, analysisOptions);
 
     var sdkResolver = new DartUriResolver(sdk);
 
     // Read the summaries.
     summaryData ??= new SummaryDataStore(options.summaryPaths,
         resourceProvider: resourceProvider, recordDependencyInfo: true);
+
     var sdkSummaryBundle = sdk.getLinkedBundle();
     if (sdkSummaryBundle != null) {
       summaryData.addBundle(null, sdkSummaryBundle);
     }
 
     var srcFactory = createSourceFactory(options,
         sdkResolver: sdkResolver,
         fileResolvers: fileResolvers,
         summaryData: summaryData,
         resourceProvider: resourceProvider);
@@ skipping 503 matching lines @@
 /// Strip out files that should not be included in the sdk sourcemap as they
 /// are implementation details that would just confuse users.
 /// Normalize sdk urls to use "dart:" for more understandable stack traces.
 Map cleanupSdkSourcemap(Map sourceMap) {
   var map = new Map.from(sourceMap);
   map['sources'] = map['sources']
       .map((url) => url.contains('/_internal/') ? null : url)
       .toList();
   return map;
 }