[J2KT] Run MakeVariablesNonNull pass also before PropagateNullability, to avoid propagating from nullable local variables which would eventually become non-null.

Also, fix infinite-loop bug in `MakeVariablesNonNull` pass.

PiperOrigin-RevId: 735470036

Author: Googler

transpiler/java/com/google/j2cl/transpiler/backend/Backend.java

@@ -729,6 +729,7 @@ public ImmutableList<Supplier<NormalizationPass>> getPassFactories(BackendOption
           FixJavaKotlinMethodOverrideMismatch::new,
           AnnotateProtobufMethodsAsKtProperties::new,
           RewriteAnnotationTypesJ2kt::new,
+          MakeVariablesNonNull::new,
           PropagateNullability::new,
           NormalizeNullLiterals::new,
           NormalizeMinValueIntegralLiterals::new,

transpiler/java/com/google/j2cl/transpiler/passes/MakeVariablesNonNull.java

@@ -15,6 +15,7 @@
  */
 package com.google.j2cl.transpiler.passes;
 
+import static com.google.common.base.Preconditions.checkState;
 import static com.google.j2cl.transpiler.ast.TypeDescriptors.isBoxedType;
 
 import com.google.j2cl.transpiler.ast.AbstractVisitor;
@@ -94,8 +95,17 @@ private void exitOperator(
 
             private void exitAssignment(Variable variable, Expression expression) {
               TypeDescriptor typeDescriptor = variable.getTypeDescriptor();
+
+              if (typeDescriptor.isPrimitive()) {
+                // Variables of primitve types can never be assigned nullable values, but when this
+                // pass is run, unboxing passes might not have run yet and we might see a
+                // potentially nullable expression.
+                return;
+              }
+
               if (!typeDescriptor.isNullable() && expression.canBeNull()) {
                 variable.setTypeDescriptor(typeDescriptor.toNullable());
+                checkState(variable.getTypeDescriptor().isNullable());
                 propagateChanges[0] = true;
               }
             }

transpiler/javatests/com/google/j2cl/readable/java/j2kt/output_kt/NullabilityInferenceWithLocalVariables.kt.txt

@@ -38,7 +38,7 @@ open class NullabilityInferenceWithLocalVariables {
   @ObjCName("testArray")
   fun testArray(): Array<String> {
    val local: String = ""
-   return arrayOf<String?>(local, "") as Array<String>
+   return arrayOf<String>(local, "")
   }
 
   @JvmStatic