Fix OOB exception in imports (microsoft#753)

* Fix microsoft#668 (partial)

* Tests

* Revert "Tests"

This reverts commit 7ffc9db.

* Suppress completion after error tokens

* Fix OOB exception

Author: Mikhail Arkhipov

src/Analysis/Ast/Impl/Analyzer/Handlers/ImportHandler.cs

@@ -70,7 +70,7 @@ private void HandleImport(ModuleName moduleImportExpression, NameExpression asNa
             // "import fob.oar.baz" is handled as fob = import_module('fob')
             if (asNameExpression != null) {
                 Eval.DeclareVariable(asNameExpression.Name, variableModule, VariableSource.Import, asNameExpression);
-            } else if (_variableModules.TryGetValue(importNames[0], out variableModule)) {
+            } else if (importNames.Count > 0 && _variableModules.TryGetValue(importNames[0], out variableModule)) {
                 var firstName = moduleImportExpression.Names[0];
                 Eval.DeclareVariable(importNames[0], variableModule, VariableSource.Import, firstName);
             }

src/LanguageServer/Test/CompletionTests.cs

@@ -863,6 +863,13 @@ public async Task NoCompletionInOpenString() {
             result.Should().HaveNoCompletion();
         }
 
+        [TestMethod, Priority(0)]
+        public async Task NoCompletionBadImportExpression() {
+            var analysis = await GetAnalysisAsync("import os,.");
+            var cs = new CompletionSource(new PlainTextDocumentationSource(), ServerSettings.completion);
+            cs.GetCompletions(analysis, new SourceLocation(1, 12)); // Should not crash.
+        }
+
         [TestMethod, Priority(0)]
         public async Task NoCompletionInComment() {