Attach experimental platform Starlark APIs to --experimental_platforms_api

Technically, this is a breaking change. However, changes being guarded by this flag were already documented as "experimental", and users can use --experimental_platforms_api to temporarily continue using this API, if they so happened to start depending on it.

RELNOTES: A number of platform-related Starlark APIs which were previously marked "experimental" are now disabled by default, and may be enabled via --experimental_platforms_api
PiperOrigin-RevId: 215963147
diff --git a/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java b/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
index 3ce2097..8246bf7 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
@@ -126,6 +126,7 @@
         "--experimental_enable_android_migration_apis=" + rand.nextBoolean(),
         "--experimental_enable_repo_mapping=" + rand.nextBoolean(),
         "--experimental_remap_main_repo=" + rand.nextBoolean(),
+        "--experimental_platforms_api=" + rand.nextBoolean(),
         "--incompatible_bzl_disallow_load_after_statement=" + rand.nextBoolean(),
         "--incompatible_depset_is_not_iterable=" + rand.nextBoolean(),
         "--incompatible_depset_union=" + rand.nextBoolean(),
@@ -168,6 +169,7 @@
         .experimentalEnableAndroidMigrationApis(rand.nextBoolean())
         .experimentalEnableRepoMapping(rand.nextBoolean())
         .experimentalRemapMainRepo(rand.nextBoolean())
+        .experimentalPlatformsApi(rand.nextBoolean())
         .incompatibleBzlDisallowLoadAfterStatement(rand.nextBoolean())
         .incompatibleDepsetIsNotIterable(rand.nextBoolean())
         .incompatibleDepsetUnion(rand.nextBoolean())
diff --git a/src/test/java/com/google/devtools/build/lib/rules/platform/ConstraintCollectionApiTest.java b/src/test/java/com/google/devtools/build/lib/rules/platform/ConstraintCollectionApiTest.java
index 18d9c53..da804aa 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/platform/ConstraintCollectionApiTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/platform/ConstraintCollectionApiTest.java
@@ -70,6 +70,7 @@
 
   @Test
   public void testGet_starlark() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_platforms_api=true");
     constraintBuilder("//foo:s1").addConstraintValue("value1").write();
     constraintBuilder("//foo:s2").addConstraintValue("value2").write();
     platformBuilder("//foo:my_platform").addConstraint("value1").addConstraint("value2").write();
diff --git a/src/test/java/com/google/devtools/build/lib/rules/platform/ConstraintTest.java b/src/test/java/com/google/devtools/build/lib/rules/platform/ConstraintTest.java
index 83e2f7e..6a66985 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/platform/ConstraintTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/platform/ConstraintTest.java
@@ -139,6 +139,7 @@
 
   @Test
   public void testConstraint_defaultValue_starlark() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_platforms_api=true");
     scratch.file(
         "constraint_default/BUILD",
         "constraint_setting(name = 'basic',",
@@ -188,6 +189,7 @@
 
   @Test
   public void testConstraint_defaultValue_notSet_starlark() throws Exception {
+    setSkylarkSemanticsOptions("--experimental_platforms_api=true");
     scratch.file("constraint_default/BUILD", "constraint_setting(name = 'basic')");
 
     scratch.file(