[dart2js] Fix folding of x is Object when x may be null.

Change-Id: Ie45dae49c936e6a163963773747a17006f9554c1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/156481
Commit-Queue: Mayank Patke <fishythefish@google.com>
Reviewed-by: Stephen Adams <sra@google.com>

pkg/compiler/lib/src/ssa/builder_kernel.dart

@@ -6308,7 +6308,7 @@ class TryCatchFinallyBuilder {
 
     AbstractValue unwrappedType = kernelBuilder._typeInferenceMap
         .getReturnTypeOf(kernelBuilder._commonElements.exceptionUnwrapper);
-    if (!kernelBuilder.options.useLegacySubtyping) {
+    if (kernelBuilder.options.useNullSafety) {
       // Global type analysis does not currently understand that strong mode
       // `Object` is not nullable, so is imprecise in the return type of the
       // unwrapper, which leads to unnecessary checks for 'on Object'.
@@ -6326,7 +6326,6 @@ class TryCatchFinallyBuilder {
     int catchesIndex = 0;
 
     void pushCondition(ir.Catch catchBlock) {
-      // `guard` is often `dynamic`, which generates `true`.
       kernelBuilder._pushIsTest(catchBlock.guard, unwrappedException,
           kernelBuilder._sourceInformationBuilder.buildCatch(catchBlock));
     }

pkg/compiler/lib/src/ssa/nodes.dart

@@ -4130,26 +4130,30 @@ AbstractBool _isTestResult(
   AbstractValueDomain abstractValueDomain = closedWorld.abstractValueDomain;
   AbstractValue subsetType = expression.instructionType;
   AbstractValue supersetType = checkedAbstractValue.abstractValue;
+  AbstractBool expressionIsNull = expression.isNull(abstractValueDomain);
 
-  if (useNullSafety &&
-      expression.isNull(abstractValueDomain).isDefinitelyTrue) {
-    if (dartType.isObject) return AbstractBool.False;
-    if (closedWorld.dartTypes.isTopType(dartType) ||
-        dartType is NullableType ||
-        dartType.isNull) {
-      return AbstractBool.True;
-    }
-    if (dartType is TypeVariableType || dartType is FunctionTypeVariable) {
-      return AbstractBool.Maybe;
-    }
-    if (dartType is LegacyType) {
-      DartType baseType = dartType.baseType;
-      if (baseType is NeverType) return AbstractBool.True;
-      if (baseType is TypeVariableType || baseType is FunctionTypeVariable) {
+  if (useNullSafety) {
+    if (expressionIsNull.isDefinitelyTrue) {
+      if (dartType.isObject) return AbstractBool.False;
+      if (closedWorld.dartTypes.isTopType(dartType) ||
+          dartType is NullableType ||
+          dartType.isNull) {
+        return AbstractBool.True;
+      }
+      if (dartType is TypeVariableType || dartType is FunctionTypeVariable) {
         return AbstractBool.Maybe;
       }
+      if (dartType is LegacyType) {
+        DartType baseType = dartType.baseType;
+        if (baseType is NeverType) return AbstractBool.True;
+        if (baseType is TypeVariableType || baseType is FunctionTypeVariable) {
+          return AbstractBool.Maybe;
+        }
+      }
+      return AbstractBool.False;
+    } else if (expressionIsNull.isPotentiallyTrue) {
+      if (dartType.isObject) return AbstractBool.Maybe;
     }
-    return AbstractBool.False;
   }
 
   if (checkedAbstractValue.isPrecise &&

pkg/compiler/lib/src/ssa/optimize.dart

@@ -1887,10 +1887,6 @@ class SsaInstructionSimplifier extends HBaseVisitor
     if (typeInput is HLoadType) {
       TypeExpressionRecipe recipe = typeInput.typeExpression;
       DartType dartType = recipe.type;
-      if (_closedWorld.dartTypes.isTopType(dartType)) {
-        return _graph.addConstantBool(true, _closedWorld);
-      }
-
       IsTestSpecialization specialization =
           SpecializedChecks.findIsTestSpecialization(
               dartType, _graph, _closedWorld);