Address initial code review concerns

packages/cassowary/lib/constant_member.dart

@@ -14,6 +14,6 @@ class ConstantMember extends _EquationMember {
   Expression asExpression() => new Expression([], this.value);
 }
 
-ConstantMember CM(num value) {
+ConstantMember cm(double value) {
   return new ConstantMember(value);
 }

packages/cassowary/lib/param.dart

@@ -12,8 +12,8 @@ class Param extends _EquationMember {
     variable._owner = this;
   }
 
-  Param.withContext(ctx)
-      : variable = new Variable(0.0),
+  Param.withContext(ctx, [double value = 0.0])
+      : variable = new Variable(value),
         context = ctx {
     variable._owner = this;
   }

packages/cassowary/lib/result.dart

@@ -5,25 +5,25 @@
 part of cassowary;
 
 class Result {
-  final bool error;
   final String message;
+  final bool error;
 
-  const Result(this.message, this.error);
+  const Result(this.message, { bool isError: true }) : error = isError;
 
-  static final Result success = const Result("Success", false);
-  static final Result unimplemented = const Result("Unimplemented", true);
+  static final Result success = const Result("Success", isError: false);
+  static final Result unimplemented = const Result("Unimplemented");
   static final Result duplicateConstraint =
-      const Result("Duplicate Constraint", true);
+      const Result("Duplicate Constraint");
   static final Result unsatisfiableConstraint =
-      const Result("Unsatisfiable Constraint", true);
+      const Result("Unsatisfiable Constraint");
   static final Result unknownConstraint =
-      const Result("Unknown Constraint", true);
+      const Result("Unknown Constraint");
   static final Result duplicateEditVariable =
-      const Result("Duplicate Edit Variable", true);
+      const Result("Duplicate Edit Variable");
   static final Result badRequiredStrength =
-      const Result("Bad Required Strength", true);
+      const Result("Bad Required Strength");
   static final Result unknownEditVariable =
-      const Result("Unknown Edit Variable", true);
+      const Result("Unknown Edit Variable");
   static final Result internalSolverError =
-      const Result("Internal Solver Error", true);
+      const Result("Internal Solver Error");
 }

packages/cassowary/lib/symbol.dart

@@ -8,7 +8,7 @@ enum _SymbolType { invalid, external, slack, error, dummy, }
 
 class _Symbol {
   final _SymbolType type;
-  int tick;
+  final int tick;
 
   _Symbol(this.type, this.tick);
 

packages/cassowary/lib/variable.dart

@@ -5,12 +5,12 @@
 part of cassowary;
 
 class Variable {
-  double value = 0.0;
+  double value;
   String name;
 
   Param _owner;
 
-  int _tick;
+  final int _tick;
   static int _total = 0;
 
   Variable(this.value) : _tick = _total++;