Simplify PackageMapUriResolver - one folder, return any file, or null.

pkg/analyzer/lib/source/package_map_resolver.dart

Index: pkg/analyzer/lib/source/package_map_resolver.dart
diff --git a/pkg/analyzer/lib/source/package_map_resolver.dart b/pkg/analyzer/lib/source/package_map_resolver.dart
index 056016fc32418d7c8833d6edd6b32a196c129d80..3eeda219d58253a953ccbdb87cf0f7c93d2ed955 100644
--- a/pkg/analyzer/lib/source/package_map_resolver.dart
+++ b/pkg/analyzer/lib/source/package_map_resolver.dart
@@ -41,6 +41,13 @@ class PackageMapUriResolver extends UriResolver {
   PackageMapUriResolver(this.resourceProvider, this.packageMap) {
     asserts.notNull(resourceProvider);
     asserts.notNull(packageMap);
+    packageMap.forEach((name, folders) {
+      if (folders.length != 1) {
+        throw new ArgumentError(
+            'Exactly one folder must be specified for a package.'
+            'Found $name = $folders');
+      }
+    });
   }
 
   @override
@@ -58,74 +65,31 @@ class PackageMapUriResolver extends UriResolver {
     // <pkgName>/<relPath>
     String pkgName = path.substring(0, index);
     String relPath = path.substring(index + 1);
-    // Try to find an existing file.
+    // If the package is known, return the corresponding file.
     List<Folder> packageDirs = packageMap[pkgName];
     if (packageDirs != null) {
-      for (Folder packageDir in packageDirs) {
-        if (packageDir.exists) {
-          Resource result = packageDir.getChild(relPath);
-          if (result is File && result.exists) {
-            return result.createSource(uri);
-          }
-        }
-      }
+      Folder packageDir = packageDirs.single;
+      File file = packageDir.getChildAssumingFile(relPath);
+      return file.createSource(uri);
     }
-    // Return a NonExistingSource instance.
-    // This helps provide more meaningful error messages to users
-    // (a missing file error, as opposed to an invalid URI error).
-    String fullPath = packageDirs != null && packageDirs.isNotEmpty
-        ? packageDirs.first.canonicalizePath(relPath)
-        : relPath;
-    return new NonExistingSource(fullPath, uri, UriKind.PACKAGE_URI);
+    return null;
   }
 
   @override
   Uri restoreAbsolute(Source source) {
     String sourcePath = source.fullName;
-    Uri bestMatch;
-    int bestMatchLength = -1;
     pathos.Context pathContext = resourceProvider.pathContext;
     for (String pkgName in packageMap.keys) {
-      List<Folder> pkgFolders = packageMap[pkgName];
-      for (int i = 0; i < pkgFolders.length; i++) {
-        Folder pkgFolder = pkgFolders[i];
-        String pkgFolderPath = pkgFolder.path;
-        if (pkgFolderPath.length > bestMatchLength &&
-            sourcePath.startsWith(pkgFolderPath + pathContext.separator)) {
-          String relPath = sourcePath.substring(pkgFolderPath.length + 1);
-          if (_isReversibleTranslation(pkgFolders, i, relPath)) {
-            List<String> relPathComponents = pathContext.split(relPath);
-            String relUriPath = pathos.posix.joinAll(relPathComponents);
-            bestMatch = Uri.parse('$PACKAGE_SCHEME:$pkgName/$relUriPath');
-            bestMatchLength = pkgFolderPath.length;
-          }
-        }
-      }
-    }
-    return bestMatch;
-  }
-
-  /**
-   * A translation from file path to package URI has just been found for
-   * using the [packageDirIndex]th element of [packageDirs], and appending the
-   * relative path [relPath].  Determine whether the translation is reversible;
-   * that is, whether translating the package URI pack to a file path will
-   * produce the file path we started with.
-   */
-  bool _isReversibleTranslation(
-      List<Folder> packageDirs, int packageDirIndex, String relPath) {
-    // The translation is reversible provided there is no prior element of
-    // [packageDirs] containing a file matching [relPath].
-    for (int i = 0; i < packageDirIndex; i++) {
-      Folder packageDir = packageDirs[i];
-      if (packageDir.exists) {
-        Resource result = packageDir.getChild(relPath);
-        if (result is File && result.exists) {
-          return false;
-        }
+      Folder pkgFolder = packageMap[pkgName][0];
+      String pkgFolderPath = pkgFolder.path;
+      if (sourcePath.startsWith(pkgFolderPath + pathContext.separator)) {
+        String relPath = sourcePath.substring(pkgFolderPath.length + 1);
+        List<String> relPathComponents = pathContext.split(relPath);
+        String relUriPath = pathos.posix.joinAll(relPathComponents);
+        return Uri.parse('$PACKAGE_SCHEME:$pkgName/$relUriPath');
       }
     }
-    return true;
+    return null;
   }
 
   /**