Make conflicting summary check configurable

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 70 matching lines @@
         new DartSdkManager(options.dartSdkPath, true), new ContentCache(),
         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);
+        resourceProvider: resourceProvider,
+        recordDependencyInfo: true,
+        disallowOverlappingSummaries: true);
 
     var sdkSummaryBundle = sdk.getLinkedBundle();
     if (sdkSummaryBundle != null) {
       summaryData.addBundle(null, sdkSummaryBundle);
     }
 
     var srcFactory = createSourceFactory(options,
         sdkResolver: sdkResolver,
         fileResolvers: fileResolvers,
         summaryData: summaryData,
@@ skipping 489 matching lines @@
     // Fall back to a relative path.
     return path.toUri(path.relative(path.fromUri(uri), from: dir)).toString();
   }
 
   for (int i = 0; i < list.length; i++) {
     list[i] = transformUri(list[i]);
   }
   map['file'] = transformUri(map['file']);
   return map;
 }