Fix CompileHelloExampleDexLarge benchmark for D8.

Extra libraries need to be put on the D8 classpath to be comparable to the R8
build. Including them in the compilation unit forces D8 to actually compile them
leading to a runtime that does not indicate a lower bound for the runtime of R8
with tree shaking.

Change-Id: I47581a13d1f27efecc522af1759e11bffd36e335
diff --git a/tools/test_helloexample.py b/tools/test_helloexample.py
index 33ca9bc..941b031 100755
--- a/tools/test_helloexample.py
+++ b/tools/test_helloexample.py
@@ -144,8 +144,12 @@
   if tool == 'd8':
     if output_mode != 'dex':
       raise ValueError('Invalid output mode for D8')
+    classpath = []
+    for cp_entry in extra:
+      classpath.extend(['--classpath', cp_entry])
     return [
-      GetCompilerPrefix(tool, output_mode, output, HELLO_JAR, lib, extra, noopt)
+      GetCompilerPrefix(
+        tool, output_mode, output, HELLO_JAR, lib, classpath, noopt)
     ]
   # The compilation is either R8 or PG.
   # Write keep rules to a temporary file.