Only mangles llvm-ar case for Darwin.

With the previous code, Android failed because `deccontext.o` does not exist to rename.
Darwin failed because `Runtime.o` wasn't found for extraction.

deps/build.py

@@ -197,24 +197,25 @@ def convert_to_thin_ar(src_fn, dest_fn, dest_obj_dn):
         cwd=v8_path)
     ar_files = ar_files.splitlines()
 
+    # llvm-ar (--clang) for Darwin (but not Android) seems to mangle
+    # the names to lowercase on extraction, while others do not.
+    ar_mangles_case = args.os == "darwin"
+
     # Extracting files one-by-one is slow, so let's group them into
     # disjoint sets and use "ar N"...
     ar_file_names = {}
     for ar_file in ar_files:
-        # llvm-ar (--clang) seems to mangle the names to lowercase on
-        # extraction, while binutils ar does not.
-        ar_file_canon = ar_file.lower() if is_clang else ar_file
+        ar_file_canon = ar_file.lower() if ar_mangles_case else ar_file
         ar_file_names.setdefault(ar_file_canon, []).append(ar_file)
 
     ar_file_groups = []
-    for ar_file_canon, ar_files in ar_file_names.items():
+    for ar_files in ar_file_names.values():
         if len(ar_file_groups) < len(ar_files):
             ar_file_groups.extend([[]] * (len(ar_files) - len(ar_file_groups)))
         for i, ar_file in enumerate(ar_files):
-            ar_file_groups[i].append(ar_file)
+            ar_file_groups[i].append(ar_file.lower() if ar_mangles_case else ar_file)
 
     for i, ar_files in enumerate(ar_file_groups):
-        # Undocumented: "ar N" is 1-based.
         subprocess_check_call(
             [
                 ar_path,
@@ -225,7 +226,7 @@ def convert_to_thin_ar(src_fn, dest_fn, dest_obj_dn):
             ] + ar_files,
             cwd=v8_path)
         for ar_file in ar_files:
-            ar_file_canon = ar_file.lower() if is_clang else ar_file
+            ar_file_canon = ar_file.lower() if ar_mangles_case else ar_file
             os.rename(os.path.join(dest_obj_dn, ar_file_canon), os.path.join(dest_obj_dn, "{}.{}.o".format(1 + i, ar_file)))
 
     if os.path.exists(dest_fn):