pass library files not directories to analysis server

editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/analysis/AnalysisServer.java

Index: editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/analysis/AnalysisServer.java
===================================================================
--- editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/analysis/AnalysisServer.java	(revision 9145)
+++ editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/analysis/AnalysisServer.java	(working copy)
@@ -139,15 +139,18 @@
    * Analyze the specified library, and keep that analysis current by tracking any changes. Also see
    * {@link Context#resolve(File, ResolveCallback)}.
    * 
-   * @param file the library file (not <code>null</code>)
+   * @param libraryFile the library file (not <code>null</code>)
    */
-  public void analyze(File file) {
-    if (!file.isAbsolute()) {
-      throw new IllegalArgumentException("File path must be absolute: " + file);
+  public void analyze(File libraryFile) {
+    if (!libraryFile.isAbsolute()) {
+      throw new IllegalArgumentException("File path must be absolute: " + libraryFile);
     }
+    if (libraryFile.isDirectory()) {
+      throw new IllegalArgumentException("Cannot analyze a directory: " + libraryFile);
+    }
     synchronized (queue) {
-      if (!libraryFiles.contains(file)) {
-        libraryFiles.add(file);
+      if (!libraryFiles.contains(libraryFile)) {
+        libraryFiles.add(libraryFile);
         // Append analysis task to the end of the queue so that any user requests take precedence
         queueAnalyzeContext();
       }