Add special completion for control-flow block exits

pkg/analysis_server/test/services/completion/statement/statement_completion_test.dart

Index: pkg/analysis_server/test/services/completion/statement/statement_completion_test.dart
diff --git a/pkg/analysis_server/test/services/completion/statement/statement_completion_test.dart b/pkg/analysis_server/test/services/completion/statement/statement_completion_test.dart
index 3d4750e7593a46a2e47669560166cabb6129b233..5cc1ff184eabb021d320c9aec1ac96fcd93291c4 100644
--- a/pkg/analysis_server/test/services/completion/statement/statement_completion_test.dart
+++ b/pkg/analysis_server/test/services/completion/statement/statement_completion_test.dart
@@ -14,6 +14,7 @@ import '../../../abstract_single_unit.dart';
 
 main() {
   defineReflectiveSuite(() {
+    defineReflectiveTests(_ControlFlowCompletionTest);
     defineReflectiveTests(_DoCompletionTest);
     defineReflectiveTests(_ForCompletionTest);
     defineReflectiveTests(_ForEachCompletionTest);
@@ -93,6 +94,177 @@ class StatementCompletionTest extends AbstractSingleUnitTest {
 }
 
 @reflectiveTest
+class _ControlFlowCompletionTest extends StatementCompletionTest {
+  test_doReturnExprLineComment() async {
+    await _prepareCompletion(
+        'return 3',
+        '''
+ex(e) {
+  do {
+    return 3//
+  } while (true);
+}
+''',
+        atEnd: true);
+    _assertHasChange(
+        'Complete control flow block',
+        '''
+ex(e) {
+  do {
+    return 3;//
+  } while (true);
+  ////
+}
+''',
+        (s) => _afterLast(s, '  '));
+  }
+
+  test_doReturnUnterminated() async {
+    await _prepareCompletion(
+        'return',
+        '''
+ex(e) {
+  do {
+    return
+  } while (true);
+}
+''',
+        atEnd: true);
+    _assertHasChange(
+        'Complete control flow block',
+        '''
+ex(e) {
+  do {
+    return;
+  } while (true);
+  ////
+}
+''',
+        (s) => _afterLast(s, '  '));
+  }
+
+  test_forEachReturn() async {
+    await _prepareCompletion(
+        'return;',
+        '''
+ex(e) {
+  for (var x in e) {
+    return;
+  }
+}
+''',
+        atEnd: true);
+    _assertHasChange(
+        'Complete control flow block',
+        '''
+ex(e) {
+  for (var x in e) {
+    return;
+  }
+  ////
+}
+''',
+        (s) => _afterLast(s, '  '));
+  }
+
+  test_forThrowUnterminated() async {
+    await _prepareCompletion(
+        'throw e',
+        '''
+ex(e) {
+  for (int i = 0; i < 3; i++) {
+    throw e
+  }
+}
+''',
+        atEnd: true);
+    _assertHasChange(
+        'Complete control flow block',
+        '''
+ex(e) {
+  for (int i = 0; i < 3; i++) {
+    throw e;
+  }
+  ////
+}
+''',
+        (s) => _afterLast(s, '  '));
+  }
+
+  test_ifThrow() async {
+    await _prepareCompletion(
+        'throw e;',
+        '''
+ex(e) {
+  if (true) {
+    throw e;
+  }
+}
+''',
+        atEnd: true);
+    _assertHasChange(
+        'Complete control flow block',
+        '''
+ex(e) {
+  if (true) {
+    throw e;
+  }
+  ////
+}
+''',
+        (s) => _afterLast(s, '  '));
+  }
+
+  test_ifThrowUnterminated() async {
+    await _prepareCompletion(
+        'throw e',
+        '''
+ex(e) {
+  if (true) {
+    throw e
+  }
+}
+''',
+        atEnd: true);
+    _assertHasChange(
+        'Complete control flow block',
+        '''
+ex(e) {
+  if (true) {
+    throw e;
+  }
+  ////
+}
+''',
+        (s) => _afterLast(s, '  '));
+  }

[Brian Wilkerson, 2017/04/21 21:45:45]

Would it be useful to have a test like

if (true) return

+
+  test_whileReturnExpr() async {
+    await _prepareCompletion(
+        '+ 4',
+        '''
+ex(e) {
+  while (true) {
+    return 3 + 4
+  }
+}
+''',
+        atEnd: true);
+    _assertHasChange(
+        'Complete control flow block',
+        '''
+ex(e) {
+  while (true) {
+    return 3 + 4;
+  }
+  ////
+}
+''',
+        (s) => _afterLast(s, '  '));
+  }
+}
+
+@reflectiveTest
 class _DoCompletionTest extends StatementCompletionTest {
   test_emptyCondition() async {
     await _prepareCompletion(