[dartdevc] Removing redundant String concat operations.

These were apparently copied from Dart2JS but never inlined in DDC, so they've been pure overhead.

Change-Id: I614a94d130248bece5ba7af8c5b9a436b1196b62
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/164682
Commit-Queue: Mark Zhou <markzipan@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>

Author: Markzipan

sdk/lib/_internal/js_dev_runtime/patch/core_patch.dart

@@ -732,8 +732,8 @@ class StringBuffer {
   @patch
   String toString() => Primitives.flattenString(_contents);
 
-  void _writeString(String str) {
-    _contents = Primitives.stringConcatUnchecked(_contents, str);
+  void _writeString(@notNull String str) {
+    _contents = JS<String>('!', '# + #', _contents, str);
   }
 
   static String _writeAll(String string, Iterable objects, String separator) {
@@ -753,8 +753,8 @@ class StringBuffer {
     return string;
   }
 
-  static String _writeOne(String string, Object? obj) {
-    return Primitives.stringConcatUnchecked(string, '$obj');
+  static String _writeOne(@notNull String string, Object? obj) {
+    return JS<String>('!', '# + #', string, '$obj');
   }
 }
 

sdk/lib/_internal/js_dev_runtime/private/foreign_helper.dart

@@ -259,15 +259,6 @@ class JS_CONST {
   const JS_CONST(this.code);
 }
 
-/**
- * JavaScript string concatenation. Inputs must be Strings.  Corresponds to the
- * HStringConcat SSA instruction and may be constant-folded.
- */
-String JS_STRING_CONCAT(String a, String b) {
-  // This body is unused, only here for type analysis.
-  return JS<String>('!', '# + #', a, b);
-}
-
 /// Same `@rest` annotation and `spread` function as in
 /// `package:js/src/varargs.dart`.
 ///

sdk/lib/_internal/js_dev_runtime/private/js_helper.dart

@@ -6,7 +6,7 @@ library dart._js_helper;
 
 import 'dart:collection';
 
-import 'dart:_foreign_helper' show JS, JS_STRING_CONCAT, JSExportName;
+import 'dart:_foreign_helper' show JS, JSExportName;
 
 import 'dart:_interceptors';
 import 'dart:_internal'
@@ -295,10 +295,6 @@ class Primitives {
     throw RangeError.range(charCode, 0, 0x10ffff);
   }
 
-  static String stringConcatUnchecked(String string1, String string2) {
-    return JS_STRING_CONCAT(string1, string2);
-  }
-
   static String flattenString(String str) {
     return JS<String>('!', "#.charCodeAt(0) == 0 ? # : #", str, str, str);
   }