Enable errorprone check to ensure hashCode is implemented.

Bug:
Change-Id: I918fd87f5d0175cdff928a8be64c6686d6689dfa
diff --git a/build.gradle b/build.gradle
index ef09f76..e189e57 100644
--- a/build.gradle
+++ b/build.gradle
@@ -21,7 +21,8 @@
     '-Xep:RemoveUnusedImports:WARN',
     '-Xep:MissingOverride:WARN',
     '-Xep:OvershadowingSubclassFields:WARN',
-    '-Xep:IntLongMath:WARN']
+    '-Xep:IntLongMath:WARN',
+    '-Xep:EqualsHashCode:WARN']
 
 apply from: 'copyAdditionalJctfCommonFiles.gradle'
 
diff --git a/src/main/java/com/android/tools/r8/bisect/BisectState.java b/src/main/java/com/android/tools/r8/bisect/BisectState.java
index b9359a3..f181022 100644
--- a/src/main/java/com/android/tools/r8/bisect/BisectState.java
+++ b/src/main/java/com/android/tools/r8/bisect/BisectState.java
@@ -104,6 +104,11 @@
       return start == o.start && end == o.end;
     }
 
+    @Override
+    public int hashCode() {
+      return 31 * start + end;
+    }
+
     private boolean verify() {
       return start <= end;
     }
diff --git a/src/main/java/com/android/tools/r8/ir/code/Position.java b/src/main/java/com/android/tools/r8/ir/code/Position.java
index a8312fb..08298bb 100644
--- a/src/main/java/com/android/tools/r8/ir/code/Position.java
+++ b/src/main/java/com/android/tools/r8/ir/code/Position.java
@@ -4,6 +4,7 @@
 package com.android.tools.r8.ir.code;
 
 import com.android.tools.r8.graph.DexString;
+import java.util.Objects;
 
 public class Position {
 
@@ -53,6 +54,14 @@
   }
 
   @Override
+  public int hashCode() {
+    int result = line;
+    result = 31 * result + Objects.hashCode(file);
+    result = 31 * result + (synthetic ? 1 : 0);
+    return result;
+  }
+
+  @Override
   public String toString() {
     if (isNone()) {
       return "--";