Add explicit-sources flag and create an ExplicitSourceResolver for analyzer.

lib/src/analyzer/context.dart

Index: lib/src/analyzer/context.dart
diff --git a/lib/src/analyzer/context.dart b/lib/src/analyzer/context.dart
index d04a6ae5b63108877a4f2e187c903fa55cf9b60f..5cfd0da12375fac001b7504c54b76d3f3a5d98cc 100644
--- a/lib/src/analyzer/context.dart
+++ b/lib/src/analyzer/context.dart
@@ -12,6 +12,7 @@ import 'package:analyzer/src/generated/source_io.dart'
     show
         CustomUriResolver,
         DartUriResolver,
+        ExplicitSourceResolver,
         FileUriResolver,
         PackageUriResolver,
         SourceFactory,
@@ -29,9 +30,13 @@ import 'multi_package_resolver.dart' show MultiPackageResolver;
 
 /// Options used to set up Source URI resolution in the analysis context.
 class AnalyzerOptions {
-  /// Custom URI mappings, such as "dart:foo" -> "path/to/foo.dart"
+  /// Custom URI mappings, such as "dart:foo" -> "path/to/foo.dart".
   final Map<String, String> customUrlMappings;
 
+  /// Whether the [customUrlMappings] are all inclusive, in which case an error
+  /// should be produced if a lookup fails.
+  final bool allInclusive;
+
   /// Package root when resolving 'package:' urls the standard way.
   final String packageRoot;
 
@@ -53,6 +58,7 @@ class AnalyzerOptions {
       this.useMockSdk: false,
       String dartSdkPath,
       this.customUrlMappings: const {},
+      this.allInclusive: false,
       this.packageRoot: 'packages/',
       this.packagePaths: const []})
       : dartSdkPath = dartSdkPath ?? getSdkDir().path;
@@ -62,6 +68,7 @@ class AnalyzerOptions {
         useMockSdk = false,
         dartSdkPath = args['dart-sdk'] ?? getSdkDir().path,
         customUrlMappings = _parseUrlMappings(args['url-mapping']),
+        allInclusive = args['all-inclusive'],
         packageRoot = args['package-root'],
         packagePaths = args['package-paths']?.split(',') ?? [];
 
@@ -82,6 +89,10 @@ class AnalyzerOptions {
               'library.dart as the source for an import of of "libraryUri".',
           allowMultiple: true,
           splitCommas: false)
+      ..addFlag('all-inclusive',
+          help: 'whether all `package:` uris are explicitly listed \n'
+              'via the url-mapping flag.',
+          defaultsTo: false)
       ..addOption('package-paths',
           help: 'use a list of directories to resolve "package:" imports');
   }
@@ -148,7 +159,7 @@ SourceFactory _createSourceFactory(AnalyzerOptions options,
     List<UriResolver> fileResolvers,
     SummaryDataStore summaryData}) {
   var resolvers = <UriResolver>[];
-  if (options.customUrlMappings.isNotEmpty) {
+  if (!options.allInclusive && options.customUrlMappings.isNotEmpty) {
     resolvers.add(new CustomUriResolver(options.customUrlMappings));
   }
   resolvers.add(sdkResolver);
@@ -162,12 +173,26 @@ SourceFactory _createSourceFactory(AnalyzerOptions options,
 }
 
 List<UriResolver> createFileResolvers(AnalyzerOptions options) {
-  return [
-    new FileUriResolver(),
-    options.useMultiPackage
-        ? new MultiPackageResolver(options.packagePaths)
-        : new PackageUriResolver([new JavaFile(options.packageRoot)])
-  ];
+  var resolvers = <UriResolver>[];
+  if (options.allInclusive) {
+    resolvers.add(new ExplicitSourceResolver(
+        _createUriToFileMap(options.customUrlMappings)));
+  }
+  resolvers.add(new FileUriResolver());
+  resolvers.add(options.useMultiPackage
+      ? new MultiPackageResolver(options.packagePaths)
+      : new PackageUriResolver([new JavaFile(options.packageRoot)]));
+  return resolvers;
+}
+
+// TODO(sigmund): delete, this was adapted from analyzer_cli, ideally this
+// should be shared code in package:analyzer.
+Map<Uri, JavaFile> _createUriToFileMap(Map<String, String> mapping) {
+  var uriToFileMap = <Uri, JavaFile>{};
+  mapping.forEach((key, value) {
+    uriToFileMap[Uri.parse(key)] = new JavaFile(value);
+  });
+  return uriToFileMap;
 }
 
 /// Creates a [DartUriResolver] that uses a mock 'dart:' library contents.