Add a function to parse metadata into a usable object.

lib/src/runner/parse_metadata.dart

Index: lib/src/runner/parse_metadata.dart
diff --git a/lib/src/runner/parse_metadata.dart b/lib/src/runner/parse_metadata.dart
new file mode 100644
index 0000000000000000000000000000000000000000..5599922bf4df8d591286df0ebe20901721400542
--- /dev/null
+++ b/lib/src/runner/parse_metadata.dart
@@ -0,0 +1,110 @@
+// Copyright (c) 2015, 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.
+
+library unittest.runner.parse_metadata;
+
+import 'dart:io';
+
+import 'package:analyzer/analyzer.dart';
+import 'package:analyzer/src/generated/ast.dart';
+import 'package:path/path.dart' as p;
+import 'package:source_span/source_span.dart';
+
+import '../backend/metadata.dart';
+import '../util/dart.dart';
+
+/// Parse the test metadata for the test file at [path].
+///
+/// Throws an [AnalysisError] if parsing fails or a [FormatException] if the
+/// test annotations are incorrect.
+Metadata parseMetadata(String path) {
+  var testOn;
+
+  var contents = new File(path).readAsStringSync();
+  var directives = parseDirectives(contents, name: path).directives;
+  var annotations = directives.isEmpty ? [] : directives.first.metadata;
+
+  // We explicitly *don't* just look for "package:unittest" imports here,
+  // because it could be re-exported from another library.
+  var prefixes = directives.map((directive) {
+    if (directive is! ImportDirective) return null;
+    if (directive.prefix == null) return null;
+    return directive.prefix.name;
+  }).where((prefix) => prefix != null).toSet();
+
+  for (var annotation in annotations) {
+    // The annotation syntax is ambiguous between named constructors and
+    // prefixed annotations, so we need to resolve that ambiguity using the
+    // known prefixes. The analyzer parses "@x.y()" as prefix "x", annotation
+    // "y", and named constructor null. It parses "@x.y.z()" as prefix "x",
+    // annotation "y", and named constructor "z".
+    var name;
+    var constructorName;
+    var identifier = annotation.name;
+    if (identifier is PrefixedIdentifier &&
+        !prefixes.contains(identifier.prefix.name) &&
+        annotation.constructorName == null) {
+      name = identifier.prefix.name;
+      constructorName = identifier.identifier.name;
+    } else {
+      name = identifier is PrefixedIdentifier
+          ? identifier.identifier.name
+          : identifier.name;
+      if (annotation.constructorName != null) {
+        constructorName = annotation.constructorName.name;
+      }
+    }
+
+    if (name != 'TestOn') continue;
+    if (constructorName != null) {
+      throw new SourceSpanFormatException(
+          'TestOn doesn\'t have a constructor named "$constructorName".',
+          _spanFor(identifier.identifier, path));
+    }
+
+    if (annotation.arguments == null) {
+      throw new SourceSpanFormatException(
+          'TestOn takes one argument.', _spanFor(annotation, path));
+    }
+
+    var args = annotation.arguments.arguments;
+    if (args.isEmpty) {
+      throw new SourceSpanFormatException(
+          'TestOn takes one argument.', _spanFor(annotation.arguments, path));
+    }
+
+    if (args.first is NamedExpression) {
+      throw new SourceSpanFormatException(
+          "TestOn doesn't take named parameters.", _spanFor(args.first, path));
+    }
+
+    if (args.length > 1) {
+      throw new SourceSpanFormatException(
+          "TestOn takes only one argument.",
+          _spanFor(annotation.arguments, path));
+    }
+
+    if (args.first is! StringLiteral) {
+      throw new SourceSpanFormatException(
+          "TestOn takes a String.", _spanFor(args.first, path));
+    }
+
+    if (testOn != null) {
+      throw new SourceSpanFormatException(
+          "Only a single TestOn annotation may be used for a given test file.",
+          _spanFor(annotation, path));
+    }
+
+    testOn = args.first.stringValue;
+  }
+
+  return new Metadata(testOn);
+}
+
+/// Creates a [SourceSpan] for [node].
+SourceSpan _spanFor(AstNode node, String path) =>
+    // Load a SourceFile from scratch here since we're only ever going to emit
+    // one error per file anyway.
+    new SourceFile(new File(path).readAsStringSync(), url: p.toUri(path))
+        .span(node.offset, node.end);