[dart2wasm] Allocate boxed bools once

When boxing bool values use cached values for true and false and avoid
allocating new boxes for bools.

Converts code like the following to allocate a boxed bool object, e.g.
in `dartifyRaw`:

    ;; bool value on stack
    local.set $var6         ;; save bool value
    i32.const 3             ;; class id
    local.get $var6         ;; get bool value
    struct.new $BoxedBool

Into:

    ;; bool value on stack
    if
      global.get $global922
    else
      global.get $global924
    end

Change-Id: Iec19d6d0d42bfe39804a6487a7ed6b53bf63b5a4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/382382
Reviewed-by: Martin Kustermann <kustermann@google.com>

pkg/dart2wasm/lib/translator.dart

@@ -875,8 +875,20 @@ class Translator with KernelNodes {
         }
       } else if (to is w.RefType) {
         // Boxing
-        ClassInfo info = classInfo[boxedClasses[from]!]!;
+        Class cls = boxedClasses[from]!;
+        ClassInfo info = classInfo[cls]!;
         assert(info.struct.isSubtypeOf(to.heapType));
+
+        if (cls == boxedBoolClass) {
+          final constantType = w.RefType(info.struct, nullable: false);
+          b.if_([], [constantType]);
+          constants.instantiateConstant(b, BoolConstant(true), constantType);
+          b.else_();
+          constants.instantiateConstant(b, BoolConstant(false), constantType);
+          b.end();
+          return;
+        }
+
         w.Local temp = b.addLocal(from);
         b.local_set(temp);
         b.i32_const(info.classId);