[dart2js] Avoid all potential holder names for local variables

'Tested' by inspection. Type variable names like `T1` are now also
'escaped', e.g. to `$T1`.  (It is quite difficult to come up with a
reasonable size repro since the original problem it requires a
deferred loaded 'part' file that refers to entities defined by several
dozen other deferred loaded part files.)

Bug: 60891
Change-Id: I32c370c591c8fa4c67a15fb633d02fba73bacf5a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435060
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

pkg/compiler/lib/src/js_backend/deferred_holder_expression.dart

diff --git a/pkg/compiler/lib/src/js_backend/deferred_holder_expression.dart b/pkg/compiler/lib/src/js_backend/deferred_holder_expression.dart
index be85c0c..01161ff 100644
--- a/pkg/compiler/lib/src/js_backend/deferred_holder_expression.dart
+++ b/pkg/compiler/lib/src/js_backend/deferred_holder_expression.dart
@@ -438,9 +438,9 @@
 
   /// Returns true if [element] is stored in the static state holder
   /// ([staticStateHolder]).  We intend to store only mutable static state
-  /// there, whereas constants are stored in 'C'. Functions, accessors,
-  /// classes, etc. are stored in one of the other objects in
-  /// [reservedGlobalObjectNames].
+  /// there, whereas constants, functions, accessors,
+  /// classes, etc. are stored in one of the other 'holder' objects.
+  // TODO(60939): Have multiple holders.
   bool _isPropertyOfStaticStateHolder(MemberEntity element) {
     // TODO(ahe): Make sure this method's documentation is always true and
     // remove the word "intend".

pkg/compiler/lib/src/js_backend/namer.dart

diff --git a/pkg/compiler/lib/src/js_backend/namer.dart b/pkg/compiler/lib/src/js_backend/namer.dart
index e9d87b2..5fa976e 100644
--- a/pkg/compiler/lib/src/js_backend/namer.dart
+++ b/pkg/compiler/lib/src/js_backend/namer.dart
@@ -1824,14 +1824,14 @@
 
   /// Returns a variable use for accessing interceptors.
   ///
-  /// This is one of the [reservedGlobalObjectNames]
+  /// This is the name of a 'holder'.
   js_ast.Expression readGlobalObjectForInterceptors() {
     return DeferredHolderExpression.forInterceptors();
   }
 
   /// Returns a variable use for accessing the class [element].
   ///
-  /// This is one of the [reservedGlobalObjectNames]
+  /// This is the name of a 'holder'.
   js_ast.Expression readGlobalObjectForClass(ClassEntity element) {
     return DeferredHolderExpression(
       DeferredHolderExpressionKind.globalObjectForClass,
@@ -1840,6 +1840,8 @@
   }
 
   /// Returns a variable use for accessing the member [element].
+  ///
+  /// This is the name of a 'holder'.
   js_ast.Expression readGlobalObjectForMember(MemberEntity element) {
     return DeferredHolderExpression(
       DeferredHolderExpressionKind.globalObjectForMember,
@@ -2002,7 +2004,6 @@
     ...javaScriptKeywords,
     ...reservedPropertySymbols,
     ...reservedGlobalSymbols,
-    ...reservedGlobalObjectNames,
     ...reservedCapitalizedGlobalSymbols,
     ...reservedGlobalHelperFunctions,
   };
@@ -2026,8 +2027,6 @@
 
   /// Names that cannot be used by local variables and parameters.
   Set<String> get _jsVariableReserved {
-    // 26 letters in the alphabet, 25 not counting I.
-    assert(reservedGlobalObjectNames.length == 25);
     assert(_sanityCheckUpperCaseNames(_jsVariableReservedCache));
     return _jsVariableReservedCache;
   }
@@ -2039,7 +2038,9 @@
   /// same name for two different inputs.
   String safeVariableName(String name) {
     name = name.replaceAll('#', '_');
-    if (_jsVariableReserved.contains(name) || name.startsWith(r'$')) {
+    if (_jsVariableReserved.contains(name) ||
+        name.startsWith(r'$') ||
+        _holderNameRE.hasMatch(name)) {
       return '\$$name';
     }
     return name;
@@ -2664,46 +2665,16 @@
   "java", "netscape", "sun",
 ];
 
-// TODO(joshualitt): Stop reserving these names after local naming is updated
-// to use frequencies.
-const List<String> reservedGlobalObjectNames = [
-  "A",
-  "B",
-  "C", // Global object for *C*onstants.
-  "D",
-  "E",
-  "F",
-  "G",
-  "H", // Global object for internal (*H*elper) libraries.
-  // I is used for used for the Isolate function.
-  "J", // Global object for the interceptor library.
-  "K",
-  "L",
-  "M",
-  "N",
-  "O",
-  "P", // Global object for other *P*latform libraries.
-  "Q",
-  "R",
-  "S",
-  "T",
-  "U",
-  "V",
-  "W", // Global object for *W*eb libraries (dart:html).
-  "X",
-  "Y",
-  "Z",
-];
+/// Part of the local variable space is reserved for 'holders'. Holder names
+/// start with an uppercase letter and are short, since they are always
+/// 'minified'. There is no fixed name for a given holder, each 'part' file has
+/// its own local name for the shared holder objects it references, starting
+/// with single letter names. In large programs there are a few thousand
+/// holders, so recognizing up to four characters (6.19M names) seems safe.
+final RegExp _holderNameRE = RegExp(r'^[A-Z].{0,3}$');
 
 const List<String> reservedGlobalHelperFunctions = ["init"];
 
-final List<String> userGlobalObjects = List.from(reservedGlobalObjectNames)
-  ..remove('C')
-  ..remove('H')
-  ..remove('J')
-  ..remove('P')
-  ..remove('W');
-
 final RegExp _identifierStartRE = RegExp(r'[A-Za-z_$]');
 final RegExp _nonIdentifierRE = RegExp(r'[^A-Za-z0-9_$]');