Fix pre-existing HintCode.UNNECESSARY_TYPE_CHECK_TRUE

This prepares for landing https://dart-review.googlesource.com/c/sdk/+/190360 into the analyzer.

PiperOrigin-RevId: 394392683
diff --git a/lib/message_format.dart b/lib/message_format.dart
index 7b31ac9..60061e1 100644
--- a/lib/message_format.dart
+++ b/lib/message_format.dart
@@ -253,8 +253,6 @@
       var patternValue = currentPattern._value;
       var patternType = currentPattern._type;
 
-      _checkAndThrow(patternType is _BlockType,
-          'The type should be a block type: $patternType');
       switch (patternType) {
         case _BlockType.string:
           result.add(patternValue as String);
@@ -410,7 +408,6 @@
     var pluralResult = Queue<String>();
     _formatBlock(option!, namedParameters, ignorePound, pluralResult);
     var plural = pluralResult.join('');
-    _checkAndThrow(plural is String, 'Empty block in plural.');
     if (ignorePound) {
       result.add(plural);
     } else {
@@ -562,8 +559,6 @@
       if (_ElementType.string == thePart._type) {
         block = _BlockTypeAndVal(_BlockType.string, thePart._value);
       } else if (_ElementType.block == thePart._type) {
-        _checkAndThrow(thePart._value is String,
-            'The value should be a string: ${thePart._value}');
         var blockType = _parseBlockType(thePart._value);
 
         switch (blockType) {
@@ -614,7 +609,6 @@
     var pos = 0;
     while (pos < parts.length) {
       var thePart = parts.elementAt(pos);
-      _checkAndThrow(thePart._value is String, 'Missing select key element.');
       var key = thePart._value;
 
       pos++;
@@ -664,7 +658,6 @@
     var pos = 0;
     while (pos < parts.length) {
       var thePart = parts.elementAt(pos);
-      _checkAndThrow(thePart._value is String, 'Missing plural key element.');
       var key = thePart._value;
 
       pos++;
@@ -722,7 +715,6 @@
     var pos = 0;
     while (pos < parts.length) {
       var thePart = parts.elementAt(pos);
-      _checkAndThrow(thePart._value is String, 'Missing ordinal key element.');
       var key = thePart._value;
 
       pos++;