Check PackageMapUriResolver constructor arguments.

pkg/analysis_server/lib/src/context_manager.dart

Index: pkg/analysis_server/lib/src/context_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index 38c3c2df3c71c67e91cf0fddd770af0c146efdb3..e9d4573cd8161e2e262cc654cbdde553c0b8bd81 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -311,26 +311,28 @@ abstract class ContextManager {
 
   /**
    * Compute the appropriate package URI resolver for [folder], and store
-   * dependency information in [info].
+   * dependency information in [info]. Return `null` if no package map can
+   * be computed.
    */
   UriResolver _computePackageUriResolver(Folder folder, _ContextInfo info) {
-    UriResolver packageUriResolver;
     if (info.packageRoot != null) {
       info.packageMapDependencies = new Set<String>();
-      packageUriResolver =
-          new PackageUriResolver([new JavaFile(info.packageRoot)]);
+      return new PackageUriResolver([new JavaFile(info.packageRoot)]);
     } else {
       beginComputePackageMap();
       PackageMapInfo packageMapInfo =
           _packageMapProvider.computePackageMap(folder);
       endComputePackageMap();
       info.packageMapDependencies = packageMapInfo.dependencies;
-      packageUriResolver =
-          new PackageMapUriResolver(resourceProvider, packageMapInfo.packageMap);
+      if (packageMapInfo.packageMap == null) {
+        return null;
+      }
+      return new PackageMapUriResolver(
+          resourceProvider,
+          packageMapInfo.packageMap);
       // TODO(paulberry): if any of the dependencies is outside of [folder],
       // we'll need to watch their parent folders as well.
     }
-    return packageUriResolver;
   }
 
   /**