Don't allow null as argument to StreamIterator constructor.

Change-Id: I10c8de2cd12660876908f719ee562006cd3f2c07
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/98001
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>
Reviewed-by: Leaf Petersen <leafp@google.com>

Author: lrhn

CHANGELOG.md

@@ -4,6 +4,11 @@
 
 ### Core library changes
 
+#### `dart:async`
+* BREAKING CHANGE: 
+  Fixes bug in `StreamIterator` which allowed constructor argument to be `null`.
+  Also allowed `await for` on a `null` stream. This is now a runtime error.
+
 #### `dart:core`
 
 * **Breaking change**: The `RegExp` interface has been extended with two new
@@ -27,6 +32,11 @@
   This change only affects implementers of the `RegExp` interface; current
   code using Dart regular expressions will not be affected.
 
+#### `dart:isolate`
+
+* BREAKING CHANGE: The `await for` allowed `null` as a stream due to a bug
+  in `StreamIterator` class. This bug has now been fixed.
+
 ## 2.3.0
 
 The focus in this release is on the new "UI-as-code" language features which

pkg/front_end/testcases/legacy.status

@@ -20,6 +20,7 @@ incomplete_field_formal_parameter: Fail # Fasta doesn't recover well
 inference/bug31436: RuntimeError # Test exercises Dart 2.0 semantics
 inference/constructors_too_many_positional_arguments: Fail
 inference/downwards_inference_annotations_locals: Fail # Issue #30031
+inference/downwards_inference_for_each: RuntimeError # Issue #36382
 inference/future_then_explicit_future: RuntimeError
 inference/generic_methods_infer_js_builtin: RuntimeError # Test attempts to access platform-private library leading to NSM.
 inference/infer_assign_to_index: Fail

pkg/front_end/testcases/strong.status

@@ -33,6 +33,7 @@ inference/constructors_infer_from_arguments_argument_not_assignable: TypeCheckEr
 inference/constructors_too_many_positional_arguments: InstrumentationMismatch # Issue #30040
 inference/do_not_infer_overridden_fields_that_explicitly_say_dynamic_infer: TypeCheckError
 inference/downwards_inference_annotations_type_variable: InstrumentationMismatch # Issue 28981
+inference/downwards_inference_for_each: RuntimeError # Issue #36382
 inference/downwards_inference_on_function_of_t_using_the_t: InstrumentationMismatch # Issue #29798
 inference/downwards_inference_on_list_literals_infer_downwards: RuntimeError
 inference/future_then_explicit_future: InstrumentationMismatch # Issue #30040

sdk/lib/async/stream_impl.dart

@@ -967,7 +967,8 @@ class _StreamIterator<T> implements StreamIterator<T> {
   /// completed.
   bool _isPaused = false;
 
-  _StreamIterator(final Stream<T> stream) : _stateData = stream;
+  _StreamIterator(final Stream<T> stream)
+      : _stateData = stream ?? (throw ArgumentError.notNull("stream"));
 
   T get current {
     if (_subscription != null && _isPaused) {