Make conflicting summary check configurable

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

Index: pkg/dev_compiler/lib/src/compiler/compiler.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/compiler.dart b/pkg/dev_compiler/lib/src/compiler/compiler.dart
index 7f1c84c1d53e369f90dddd6a093fff39cfcc4e9a..c07c31c11f532b0d8e462482e4750a5d2ad6a7b2 100644
--- a/pkg/dev_compiler/lib/src/compiler/compiler.dart
+++ b/pkg/dev_compiler/lib/src/compiler/compiler.dart
@@ -88,7 +88,9 @@ class ModuleCompiler {
 
     // 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) {