[test_runner] Fix RangeError when parsing static errors

Previously it tested if it was at the end of file (defined as
_currentLine >= _lines.length), if it was not it would try to read the
_next_ line (_peek(1)), meaning that if _currentLine = _lines.length - 1
it would _peek(1) and try to read _lines[lines.length] and crash with a
range error. This CL fixes it by actually checking if it can safely peek
the amount it wants to.

Change-Id: I2e66c539e823f0b39bd0da39e2b2277a9e6c9501
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150624
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>

pkg/test_runner/lib/src/static_error.dart

@@ -456,7 +456,7 @@ class _ErrorExpectationParser {
   _ErrorExpectationParser(String source) : _lines = source.split("\n");
 
   List<StaticError> _parse() {
-    while (!_isAtEnd) {
+    while (_canPeek(0)) {
       var sourceLine = _peek(0);
 
       var match = _caretLocationRegExp.firstMatch(sourceLine);
@@ -504,7 +504,7 @@ class _ErrorExpectationParser {
 
     var startLine = _currentLine;
 
-    while (!_isAtEnd) {
+    while (_canPeek(1)) {
       var match = _errorMessageRegExp.firstMatch(_peek(1));
       if (match == null) break;
 
@@ -516,7 +516,7 @@ class _ErrorExpectationParser {
       _advance();
 
       // Consume as many additional error message lines as we find.
-      while (!_isAtEnd) {
+      while (_canPeek(1)) {
         var nextLine = _peek(1);
 
         // A location line shouldn't be treated as part of the message.
@@ -585,7 +585,7 @@ class _ErrorExpectationParser {
         markerEndLine: _currentLine));
   }
 
-  bool get _isAtEnd => _currentLine >= _lines.length;
+  bool _canPeek(int offset) => _currentLine + offset < _lines.length;
 
   void _advance() {
     _currentLine++;

pkg/test_runner/test/update_errors_test.dart

@@ -287,6 +287,17 @@ someBadCode();
 /\/ [cfe] Wrong 1.
 """);
 
+  // Don't crash with RangeError.
+  expectUpdate("""
+x
+// [error line 1, column 1, length 0]
+// [cfe] Whatever""", errors: [
+    makeError(line: 1, column: 1, length: 0, cfeError: "Foo"),
+  ], expected: """
+x
+// [error line 1, column 1, length 0]
+// [cfe] Foo""");
+
   regression();
 }