Relax the expected dex file version 51 -> 50.

Bug:
Change-Id: I3768be09dee14c08c0ed78c1712bec3d54b6d580
diff --git a/src/main/java/com/android/tools/r8/dex/Constants.java b/src/main/java/com/android/tools/r8/dex/Constants.java
index 03f2ad5..77a82df 100644
--- a/src/main/java/com/android/tools/r8/dex/Constants.java
+++ b/src/main/java/com/android/tools/r8/dex/Constants.java
@@ -15,8 +15,8 @@
   public static final int MIN_VDEX_VERSION = 10;
   public static final int MAX_VDEX_VERSION = 11;
 
-  // We apply Java 7 class file constraints on DEX files.
-  public static final int CORRESPONDING_CLASS_FILE_VERSION = 51;
+  // We apply Java 6 class file constraints on DEX files.
+  public static final int CORRESPONDING_CLASS_FILE_VERSION = 50;
 
   public static final int DEX_MAGIC_SIZE = 8;