support part file completions

pkg/analysis_server/lib/src/services/completion/dart_completion_manager.dart

Index: pkg/analysis_server/lib/src/services/completion/dart_completion_manager.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart_completion_manager.dart b/pkg/analysis_server/lib/src/services/completion/dart_completion_manager.dart
index 5426d294ad60a3b8deefd78f3bc2b8e9779ef4d0..2a5acd7ddca12a2d12bb83891dbca47e50eb26b3 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart_completion_manager.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart_completion_manager.dart
@@ -189,9 +189,13 @@ class DartCompletionManager extends CompletionManager {
    * compilation unit is never going to be resolved.
    */
   Future<CompilationUnit> waitForAnalysis() {
+    List<Source> libraries = context.getLibrariesContaining(source);
+    if (libraries == null || libraries.length == 0 || libraries[0] == null) {

[Paul Berry, 2014/12/18 18:37:02]

Are all of these possibilities expected to arise in practice?  My understanding
of the contract of getLibrariesContainingSource() is that it always returns a
non-null list containing non-null elements, so the only case which can actually
arise is libraries.length == 0.  I'm fine with defensive programming, but we
should include an assert statement inside the body of the if statement so that
(a) if getLibrariesContainingSource() ever breaks its contract we'll find out,
and (b) future readers of the code won't draw incorrect conclusions about the
contract.

Accordingly, I'd recommend adding this assertion:

assert(libraries != null && libraries.length == 0);

[danrubel, 2014/12/19 18:46:09]

The dartdoc does not clearly state that it will not return a null array or null
element in the array. That said, I like your point that those should not happen
in practice and thus should be coded as assert statements. Done.

+      return new Future.value(null);
+    }
     return context.computeResolvedCompilationUnitAsync(
         source,
-        source).catchError((_) {
+        libraries[0]).catchError((_) {
       // This source file is not scheduled for analysis, so a resolved
       // compilation unit is never going to get computed.
       return null;