fix dartanalyzer find options in bazel workspace

pkg/analyzer_cli/lib/src/driver.dart

 // 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 analyzer_cli.src.driver;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io' as io;
 
@@ skipping 721 matching lines @@
       ResourceProvider resourceProvider, CommandLineOptions options) {
     if (options.analysisOptionsFile != null) {
       file_system.File file =
           resourceProvider.getFile(options.analysisOptionsFile);
       if (!file.exists) {
         printAndFail('Options file not found: ${options.analysisOptionsFile}',
             exitCode: ErrorSeverity.ERROR.ordinal);
       }
     }
 
+    String contextRoot;
+    if (options.sourceFiles.isEmpty) {
+      contextRoot = path.current;
+    } else {
+      contextRoot = options.sourceFiles[0];
+      if (!path.isAbsolute(contextRoot)) {
+        contextRoot = path.absolute(contextRoot);
+      }
+    }
     AnalysisOptionsImpl contextOptions = new ContextBuilder(
             resourceProvider, null, null,
             options: options.contextBuilderOptions)
-        .getAnalysisOptions(options.sourceFiles.isNotEmpty
-            ? options.sourceFiles[0]
-            : path.current);
+        .getAnalysisOptions(contextRoot);
 
     contextOptions.trackCacheDependencies = false;
     contextOptions.disableCacheFlushing = options.disableCacheFlushing;
     contextOptions.hint = !options.disableHints;
     contextOptions.generateImplicitErrors = options.showPackageWarnings;
     contextOptions.generateSdkErrors = options.showSdkWarnings;
 
     return contextOptions;
   }
 
@@ skipping 141 matching lines @@
     for (var package in packages) {
       var packageName = path.basename(package.path);
       var realPath = package.resolveSymbolicLinksSync();
       result[packageName] = [
         PhysicalResourceProvider.INSTANCE.getFolder(realPath)
       ];
     }
     return result;
   }
 }