Near complete rewrite of the ProcessCommonJsModules pass.

Avoids creating aliases for imports and exports thus removing the dependency on the early inlining pass.
Only traverses the AST twice.

Closes #2085

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136504299

src/com/google/javascript/jscomp/DiagnosticGroups.java

@@ -536,7 +536,8 @@ public DiagnosticGroup forName(String name) {
           CheckSuspiciousCode.SUSPICIOUS_IN_OPERATOR,
           CheckSuspiciousCode.SUSPICIOUS_INSTANCEOF_LEFT_OPERAND,
           CheckSuspiciousCode.SUSPICIOUS_NEGATED_LEFT_OPERAND_OF_IN_OPERATOR,
-          TypeCheck.DETERMINISTIC_TEST);
+          TypeCheck.DETERMINISTIC_TEST,
+          ProcessCommonJSModules.SUSPICIOUS_EXPORTS_ASSIGNMENT);
 
   public static final DiagnosticGroup FUNCTION_PARAMS =
       DiagnosticGroups.registerGroup(

src/com/google/javascript/jscomp/NodeTraversal.java

@@ -694,7 +694,7 @@ private void traverseClass(Node n, Node parent) {
       traverseBranch(className, n);
     }
 
-    final Node extendsClause = className.getNext();
+    final Node extendsClause = n.getSecondChild();
     final Node body = extendsClause.getNext();
 
     // Extends