Fix a couple of covariant overrides in Expression. (#5257)

It was widening the return type of a couple of operators compared to
what EquationMember declared it should return. Since it did in fact
always return an Expression, the widening wasn't needed.

This just tightens them to match the base class.

packages/flutter/lib/src/cassowary/expression.dart

@@ -148,7 +148,7 @@ class Expression extends EquationMember {
   }
 
   @override
-  EquationMember operator *(EquationMember m) {
+  Expression operator *(EquationMember m) {
     _Multiplication args = _findMulitplierAndMultiplicand(m);
 
     if (args == null) {
@@ -162,7 +162,7 @@ class Expression extends EquationMember {
   }
 
   @override
-  EquationMember operator /(EquationMember m) {
+  Expression operator /(EquationMember m) {
     if (!m.isConstant) {
       throw new ParserException(
           'The divisor was not a constant expression', <EquationMember>[this, m]);
@@ -187,7 +187,7 @@ class Expression extends EquationMember {
     return null;
   }
 
-  EquationMember _applyMultiplicand(double m) {
+  Expression _applyMultiplicand(double m) {
     List<Term> newTerms = terms.fold(
       new List<Term>(),
       (List<Term> list, Term term) {